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How to Use KVM Virtualization? A Complete Guide

KVM (Kernel-based Virtual Machine) virtualization is a powerful virtualization technology built directly into the Linux kernel. It allows you to create and run multiple virtual machines (VMs) on a single physical server, which significantly increases resource utilization and reduces costs. This guide, written by an experienced system administrator, will walk you through all the steps of setting up and using KVM, from installation to performance optimization. We will thoroughly cover the installation of necessary packages, creation and configuration of virtual machines, their management, and troubleshooting common issues. Whether you are new to virtualization or an experienced user, this guide will provide you with all the necessary knowledge and practical skills for successful KVM usage.

Table of Contents

Installing KVM and Necessary Packages

Как использовать KVM виртуализацию? - Installing KVM packages on Ubuntu/Debian/CentOS/RHEL. Show package manager commands for each OS.
The first step is to install the necessary KVM packages. The installation process varies slightly depending on the Linux distribution you are using. Let’s look at examples for Ubuntu/Debian and CentOS/RHEL.

Installation on Ubuntu/Debian To install KVM and related tools on Ubuntu or Debian, run the following commands in the terminal:

sudo apt update
sudo apt install -y qemu-kvm libvirt-daemon-system libvirt-clients bridge-utils virtinst
* qemu-kvm: The core package for KVM emulation. * libvirt-daemon-system: The libvirtd daemon, which manages virtual machines. * libvirt-clients: Client tools for managing libvirtd. * bridge-utils: Utilities for creating network bridges. * virtinst: Tools for creating virtual machines from the command line.
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After installation, you need to check if virtualization is supported on your processor. Run the following command:

egrep -c '(vmx|svm)' /proc/cpuinfo
If the command output is greater than 0, then virtualization is supported.

Next, add your user to the libvirt group to gain access to managing virtual machines without using sudo:

sudo usermod -a -G libvirt $USER
newgrp libvirt
Now check the status of the libvirtd service:

systemctl status libvirtd
You should see that the service is running and active. If not, start it:

sudo systemctl start libvirtd
sudo systemctl enable libvirtd
Installation on CentOS/RHEL To install KVM on CentOS or RHEL, run the following commands:

sudo yum install -y qemu-kvm libvirt virt-install bridge-utils
* qemu-kvm: The core package for KVM emulation. * libvirt: The package containing the libvirtd daemon and client tools. * virt-install: Tools for creating virtual machines from the command line. * bridge-utils: Utilities for creating network bridges.

Checking for virtualization support on the processor is similar to Ubuntu/Debian:

egrep -c '(vmx|svm)' /proc/cpuinfo
Add your user to the libvirt group:

sudo usermod -a -G libvirt $USER
newgrp libvirt
Check the status of the libvirtd service:

sudo systemctl status libvirtd
If the service is not running, start and enable it:

sudo systemctl start libvirtd
sudo systemctl enable libvirtd
**Tip:** Make sure that virtualization is enabled in your server’s BIOS (Intel VT-x or AMD-V). Without this, KVM will not be able to work.

**Warning:** Incorrect configuration of network bridges can lead to loss of network connectivity on the host system. Be careful when configuring the network!

Virtualization allows you to effectively use computing resources, reducing the need for physical servers and simplifying infrastructure management.

Dmitry Ivanov, System Architect
Now that KVM is installed and configured, let’s move on to creating your first virtual machine.

Creating Your First Virtual Machine

Как использовать KVM виртуализацию? - Screenshot of virt-manager GUI or command line creation of a VM.
There are several ways to create a KVM virtual machine: using the graphical interface (virt-manager) or from the command line (virt-install). Let’s look at both options.

Creating a VM using virt-manager Virt-manager is a graphical tool for managing virtual machines. If it is not already installed, install it:

sudo apt install -y virt-manager   # Ubuntu/Debian
sudo yum install -y virt-manager   # CentOS/RHEL
Run virt-manager:

virt-manager
In the main window, click the «Create a new virtual machine» button. Follow the wizard instructions. You will need to specify:

  • The name of the virtual machine.
  • The method of installing the Your VPS Operating System" class="internal-post-link">operating system (ISO image, network installation, etc.).
  • The amount of allocated RAM and the number of virtual processors.
  • The size of the virtual machine disk.
  • Network settings.
**Example:** Creating an Ubuntu 20.04 virtual machine from an ISO image.

Creating a VM using virt-install Virt-install is a command-line utility for creating virtual machines. It provides more flexibility and automation possibilities.

**Example:** Creating a CentOS 7 virtual machine from an ISO image:

virt-install \
--name=centos7-vm \
--ram=2048 \
--vcpus=2 \
--os-variant=centos7.0 \
--cdrom=/path/to/CentOS-7-x86_64-DVD-2009.iso \
--disk path=/var/lib/libvirt/images/centos7-vm.img,size=20 \
--network bridge=virbr0,model=virtio \
--graphics vnc,listen=0.0.0.0 --noautoconsole
Let’s break down the command parameters:

  • --name: The name of the virtual machine (centos7-vm).
  • --ram: The amount of allocated RAM in MB (2048).
  • --vcpus: The number of virtual processors (2).
  • --os-variant: The operating system variant (centos7.0). Specify the most appropriate option for your OS. A list of available options can be obtained using the command osinfo-query os.
  • --cdrom: The path to the ISO image of the installation disk (/path/to/CentOS-7-x86_64-DVD-2009.iso). Replace with your path.
  • --disk: Virtual machine disk parameters. path – the path to the disk image file (/var/lib/libvirt/images/centos7-vm.img), size – the disk size in GB (20).
  • --network: Network settings. bridge=virbr0 – connection to the virbr0 network bridge (default), model=virtio – using the virtio driver for the network card (provides better performance).
  • --graphics: Graphics interface settings. vnc,listen=0.0.0.0 – using VNC to connect to the virtual machine. listen=0.0.0.0 means that the VNC server will listen on all IP addresses. --noautoconsole disables automatic connection to the console.
After executing the command, the operating system installation process will begin. You can connect to the virtual machine via a VNC client, specifying the IP address of the host system and the VNC port (default 5900). If you are using a firewall, make sure the VNC port is open.

sudo firewall-cmd --permanent --add-port=5900/tcp
sudo firewall-cmd --reload
**Tip:** Use virtio drivers for disks and network cards in virtual machines. They provide significantly better performance compared to emulated drivers.

**Warning:** Allocating too many resources to a virtual machine can reduce the performance of the host system.

Proper resource planning is key to successful virtualization. Consider the needs of both the host system and the virtual machines.

Elena Smirnova, DevOps Engineer
Now that you have created a virtual machine, let’s move on to managing them.

Managing Virtual Machines

KVM virtual machines are managed using the virsh utility (command-line interface for libvirt). It provides a wide range of commands for performing various operations.

Basic virsh commands * **List virtual machines:**

virsh list --all
This command displays a list of all virtual machines, including running and stopped ones.

* **Start a virtual machine:**

virsh start centos7-vm
Replace centos7-vm with the name of your virtual machine.

* **Shut down a virtual machine:**

virsh shutdown centos7-vm
This command sends an ACPI shutdown signal to the virtual machine, allowing it to shut down correctly.

* **Forcefully stop a virtual machine:**

virsh destroy centos7-vm
This command immediately terminates the virtual machine, which may result in data loss. Use it only as a last resort.

* **Reboot a virtual machine:**

virsh reboot centos7-vm
* **Connect to the virtual machine console:**

virsh console centos7-vm
This command allows you to connect to the text console of the virtual machine. To exit the console, press Ctrl+].

* **Get information about a virtual machine:**

virsh dominfo centos7-vm
This command displays detailed information about the virtual machine, including name, ID, state, CPU and memory usage.

* **Edit the virtual machine configuration:**

virsh edit centos7-vm
This command opens the virtual machine configuration file in a text editor. Changing the configuration can affect the operation of the virtual machine, so be careful.

**Example of a virtual machine configuration file (centos7-vm.xml):**

<domain type='kvm' id='1'>
  <name>centos7-vm</name>
  <uuid>9690f123-58c4-4b1e-b917-7b76474930a4</uuid>
  <memory unit='KiB'>2097152</memory>
  <currentMemory unit='KiB'>2097152</currentMemory>
  <vcpu placement='static'>2</vcpu>
  <os>
    <type arch='x86_64' machine='pc-i440fx-rhel7.6.0'>hvm</type>
    <boot dev='cdrom'/>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
    <apic/>
    <vmport state='off'/>
  </features>
  <cpu mode='host-model' check='partial'>
    <model fallback='allow'/>
  </cpu>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/bin/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/centos7-vm.img'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </disk>
    <disk type='file' device='cdrom'>
      <driver name='qemu' type='raw'/>
      <source file='/path/to/CentOS-7-x86_64-DVD-2009.iso'/>
      <target dev='hdc' bus='ide'/>
      <readonly/>
      <address type='drive' controller='0' bus='1' target='0' unit='0'/>
    </disk>
    <interface type='bridge'>
      <mac address='52:54:00:7f:b8:e9'/>
      <source bridge='virbr0'/>
      <model type='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>
    <serial type='pty'>
      <target port='0'/>
    </serial>
    <console type='pty'>
      <target type='serial' port='0'/>
    </console>
    <input type='tablet' bus='usb'/>
    <input type='mouse' bus='ps2'/>
    <graphics type='vnc' port='5900' autoport='no' listen='0.0.0.0'>
      <listen type='address' address='0.0.0.0'/>
    </graphics>
    <sound model='ich6'/>
    <video>
      <model type='cirrus' vram='16384' heads='1' primary='yes'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>
    <memballoon model='virtio'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </memballoon>
  </devices>
</domain>
Replace /path/to/CentOS-7-x86_64-DVD-2009.iso with the actual path to your ISO image.

**Tip:** Create backups of virtual machine configuration files before making changes.

**Warning:** Incorrect virtual machine configuration can lead to its failure.

Automating virtual machine management tasks can significantly reduce administrative costs and increase infrastructure reliability.

Red Hat KVM Documentation
Now let’s look at configuring the network for virtual machines.

Networking and Connecting to VMs

Network configuration is an important aspect of virtualization. KVM supports various network configuration options, including bridged networking and NAT (Network Address Translation).

Bridged Networking Bridged networking allows the virtual machine to connect directly to the physical network. The virtual machine receives an IP address from the same subnet as the host system.

By default, libvirt creates a network bridge virbr0 with NAT. To use bridged networking, you need to create a bridge interface on the host system and assign it the IP address of the physical interface.

**Example of configuring a network bridge on Ubuntu/Debian:**

Suppose you have a network interface enp0s3 with the IP address 192.168.1.100. Create the file /etc/network/interfaces.d/br0.cfg:

auto br0
iface br0 inet static
    address 192.168.1.100
    netmask 255.255.255.0
    gateway 192.168.1.1
    bridge_ports enp0s3
    bridge_stp off
    bridge_fd 0
    bridge_maxwait 0
Let’s break down the parameters:

  • auto br0: Enables the br0 interface when the system boots.
  • iface br0 inet static: Specifies that the br0 interface uses a static IP address.
  • address: The IP address of the br0 interface (192.168.1.100).
  • netmask: The subnet mask (255.255.255.0).
  • gateway: The IP address of the gateway (192.168.1.1).
  • bridge_ports enp0s3: Specifies that the enp0s3 interface is a member of the br0 bridge.
  • bridge_stp off: Disables the STP (Spanning Tree Protocol) protocol.
  • bridge_fd 0: Sets the forward delay time to 0 seconds.
  • bridge_maxwait 0: Sets the maximum wait time before activating the bridge to 0 seconds.
Then edit the /etc/network/interfaces file, commenting out or deleting the settings for the enp0s3 interface:

#auto enp0s3
#iface enp0s3 inet static
#    address 192.168.1.100
#    netmask 255.255.255.0
#    gateway 192.168.1.1
Restart the network interfaces:

sudo ifdown enp0s3
sudo ifup br0
Now virtual machines connected to the br0 bridge will receive IP addresses from the same subnet as the host system.

**Example of configuring a network bridge on CentOS/RHEL:**

Create the file /etc/sysconfig/network-scripts/ifcfg-br0:

DEVICE=br0
TYPE=Bridge
BOOTPROTO=static
IPADDR=192.168.1.100
NETMASK=255.255.255.0
GATEWAY=192.168.1.1
ONBOOT=yes
Edit the file /etc/sysconfig/network-scripts/ifcfg-enp0s3 (replace enp0s3 with the name of your physical interface):

DEVICE=enp0s3
TYPE=Ethernet
BOOTPROTO=none
ONBOOT=yes
BRIDGE=br0
Restart the network service:

sudo systemctl restart network
NAT (Network Address Translation) NAT allows virtual machines to access the Internet through the IP address of the host system. Virtual machines receive IP addresses from a private subnet created by libvirt (default 192.168.122.0/24).

To use NAT, it is sufficient to connect the virtual machine to the network bridge virbr0 (default). Libvirt automatically configures NAT and routing.

**Checking the network connection:**

Inside the virtual machine, run the command:

ping 8.8.8.8
If the ping is successful, it means that the virtual machine has access to the Internet.

**Tip:** Use DNSmasq to automatically assign IP addresses to virtual machines in the NAT network.

**Warning:** Incorrect routing configuration can lead to virtual machines being inaccessible from the outside.

Proper network configuration is the key to a stable and secure virtual infrastructure.

Ubuntu KVM Documentation
Now let’s move on to optimizing KVM performance.

Optimizing KVM Performance

Optimizing KVM performance is an important step to getting the most out of virtualization. There are several ways to improve the performance of virtual machines.

Using VirtIO Drivers VirtIO is a set of virtualization drivers designed to improve the performance of virtual machines. They provide direct communication between the virtual machine and the host system, minimizing emulation overhead.

Make sure that when creating a virtual machine, you use VirtIO drivers for disks and network cards. In most Linux distributions, VirtIO drivers are installed by default. If this is not the case, install them manually:

sudo apt install -y virtio-blk virtio-net  # Ubuntu/Debian
sudo yum install -y virtio-win            # CentOS/RHEL (for Windows VM)
Allocating CPU and Memory Resources Proper allocation of CPU and memory resources plays an important role in the performance of virtual machines. Allocate sufficient resources to each virtual machine, but do not overdo it so as not to reduce the performance of the host system.

* **CPU:** Allocate each virtual machine as many virtual processors as it needs to perform tasks. Start with a small number and increase it if necessary. * **Memory:** Allocate each virtual machine enough RAM so that it can work without using swap. Avoid allocating too much memory, as this can reduce the performance of the host system.

You can change the amount of allocated CPU and memory resources using the virsh edit command or in the virt-manager graphical interface.

Using KVM-clock KVM-clock is a virtual time source designed to improve time accuracy in virtual machines. It provides more stable and accurate time synchronization compared to emulated time sources.

To enable KVM-clock, add the following lines to the virtual machine configuration file (using virsh edit):

<clock offset='utc'>
    <timer name='kvmclock' present='yes'/>
</clock>
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