Master KVM: Deploy and Manage Virtual Machines on Linux with Libvirt

Introduction

KVM (Kernel Virtual Machine) is a virtualization solution originally developed by Qumranet, merged into the Linux kernel since version 2.6.20, and supported by Red Hat after acquiring Qumranet in 2008. This article explains how to deploy and manage KVM.

KVM Overview

Features

KVM runs on x86 CPUs with Intel VT or AMD‑V, uses the kvm.ko kernel module (or kvm‑intel.ko / kvm‑amd.ko) and a modified QEMU ( qemu‑kvm) as the user‑space controller. Memory management relies on the Linux kernel, and KVM can use any storage supported by Linux. Performance reaches about 95 % of native speed, and it scales to many CPUs and large memory.

Architecture

KVM consists of a kernel module and QEMU that emulates hardware; each VM is a regular Linux process managed by the kernel.

Platform Deployment

Hardware detection

Check CPU flags for virtualization support: egrep --color '(vmx|svm)' /proc/cpuinfo If the output contains vmx (Intel) or svm (AMD), the hardware supports KVM.

Load modules:

modprobe kvm

modprobe kvm_intel   # or kvm_amd

lsmod | grep kvm

Install management tools

yum install qemu-kvm qemu-kvm-tools -y

ln -s /usr/libexec/qemu-kvm /usr/sbin/

qemu‑kvm command options

Common options include:

-name name – set VM name

-M machine – select machine type

-m megs – RAM size

-cpu model – CPU model

-smp … – SMP configuration

-drive … – define disk devices

-net … – network configuration

Create disk image

qemu-img create -f qcow2 -o size=50G,preallocation=metadata /kvm/images/centos6.qcow2

Configure bridge

vim /etc/sysconfig/network-scripts/ifcfg-br0

Set DEVICE=br0, TYPE=Bridge, ONBOOT=yes, BOOTPROTO=none, IPADDR=172.16.10.124, PREFIX=16, GATEWAY=172.16.0.1, DNS1=172.16.0.1 vim /etc/sysconfig/network-scripts/ifcfg-eth0 Set DEVICE=eth0, TYPE=Ethernet, ONBOOT=yes, BRIDGE=br0

service network restart

Bridge scripts

#!/bin/bash
switch=br0
if [ -n "$1" ]; then
  ip link set $1 up
  sleep 1
  brctl addif $switch $1
  exit 0
else
  echo "Error: No Interface."
  exit 1
fi

#!/bin/bash
switch=br0
if [ -n "$1" ]; then
  brctl delif $switch $1
  ip link set $1 down
  exit 0
else
  echo "Error: No Interface."
  exit 1
fi

chmod -R +x /kvm/script/

Install a VM with qemu‑kvm

yum install tigervnc -y

qemu-kvm -name "centos6.6" \
 -m 512 -smp 2 \
 -drive file=/kvm/images/centos6.qcow2,media=disk,format=qcow2 \
 -drive file=CentOS-6.6-x86_64-bin-DVD1.iso,media=cdrom \
 -net nic -net tap,ifname=vnet0,script=/kvm/script/qemu-ifup,downscript=/kvm/script/qemu-ifdown \
 -boot order=dc,once=d

VNC server runs on ::1:5900; connect with vncviewer :5900.

Libvirt‑based management

Libvirt provides an API and CLI tools to simplify VM lifecycle management. Install components:

yum install libvirt virt-manager virt-viewer python-virtinst -y

Start the daemon: service libvirtd start Create a bridge with libvirt:

virsh iface-bridge eth0 br0

virt‑install

virt‑install creates VMs via libvirt. Common options:

-n NAME – VM name

-r MEMORY – RAM in MB

--vcpus … – CPU count

--cpu … – CPU model

-c CDROM – installation media

--network … – network configuration

--disk … – storage definition

--boot … – boot order

--autostart – start VM on host boot

Example virt‑install command

virt-install -n "centos6.6" \
 --vcpus=2 -r 512 -c CentOS-6.6-x86_64-bin-DVD1.iso \
 --disk path=/kvm/images/centos6.6.qcow2,bus=virtio,size=50,sparse \
 --network bridge=br0,model=virtio --force

virt‑manager

virt‑manager is a Python‑based graphical front‑end that uses libvirt to manage VMs. After mounting the installation ISO and configuring an HTTP repository, start the GUI with virt-manager & and create a new VM, optionally using network installation.

Conclusion

KVM can be managed directly with qemu‑kvm commands or more conveniently through libvirt tools such as virt‑install and virt‑manager; the choice depends on personal preference.