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QEMU/KVM Installation Guide

QEMU is a generic open-source machine emulator and virtualizer. When combined with KVM (Kernel-based Virtual Machine), it provides near-native performance for virtual machines on Linux.

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About QEMU and KVM

QEMU is a flexible emulator that can:
  • Emulate different CPU architectures
  • Run virtual machines with various operating systems
  • Work with or without hardware acceleration
KVM is a Linux kernel module that:
  • Provides hardware virtualization support
  • Allows QEMU to run VMs at near-native speed
  • Requires CPU virtualization extensions (Intel VT-x or AMD-V)

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Prerequisites

System Requirements
  • Linux operating system
  • 64-bit CPU with virtualization extensions (Intel VT-x or AMD-V)
  • Sudo/root access for installation
  • Basic command-line knowledge

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Checking Virtualization Support

Before installing KVM, verify your CPU supports virtualization extensions: 
egrep '(vmx|svm)' /proc/cpuinfo
If this command returns no output, your CPU doesn’t support virtualization extensions or they’re disabled in BIOS. You can still use QEMU without KVM, but performance will be significantly slower.
  • vmx: Intel VT-x virtualization technology
  • svm: AMD-V virtualization technology
If you see multiple lines with these flags, your CPU supports virtualization.

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Installing QEMU and KVM

1

Install KVM

If your CPU supports virtualization, install KVM:
sudo apt install kvm
If the check in the previous step returned nothing, skip KVM installation. You can still use QEMU without it.
2

Install QEMU

Install the QEMU emulator:
sudo apt install qemu
This installs the core QEMU packages for various architectures.
3

Verify Installation

Check that QEMU is installed correctly:
qemu-system-x86_64 --version

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Testing QEMU

You can test QEMU with pre-built images from the QEMU Testing wiki.
# Download a test image
wget http://wiki.qemu.org/download/linux-0.2.img.bz2
bunzip2 linux-0.2.img.bz2

# Run the test image
qemu-system-x86_64 linux-0.2.img

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Creating a Virtual Disk

QEMU supports various disk image formats:
  • raw: Simple binary image (compatible but large)
  • qcow2: QEMU’s native format (flexible, supports snapshots)
  • vmdk: VMware format
  • vdi: VirtualBox format

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Create a qcow2 disk:

qemu-img create -f qcow2 debian9.img 3G
  • qemu-img create: Creates a new disk image
  • -f qcow2: Specifies the format (qcow2 is recommended)
  • debian9.img: Name of the image file
  • 3G: Size of the virtual disk (3 gigabytes)

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Installing an Operating System

1

Download an ISO

Download an operating system ISO. For example, Debian:
wget https://cdimage.debian.org/debian-cd/current-live/amd64/iso-hybrid/debian-live-12.0.0-amd64-xfce.iso
2

Start Installation

Boot the VM with the ISO attached:
qemu-system-x86_64 -m 2048 -hda debian9.img -cdrom debian-live-12.0.0-amd64-xfce.iso -boot d
  • qemu-system-x86_64: 64-bit x86 emulator
  • -m 2048: Allocate 2048 MB (2 GB) RAM
  • -hda debian9.img: Attach the virtual disk as the primary hard drive
  • -cdrom file.iso: Attach the ISO as a CD-ROM
  • -boot d: Boot from CD-ROM (d = CD-ROM drive)
3

Complete Installation

Follow the OS installation process as you would on a physical machine.When complete, shut down the VM normally.

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Running the Installed System

After installation, boot from the hard disk: 
qemu-system-x86_64 -m 2048 -hda debian9.img
The -boot d option is removed, so QEMU boots from the hard disk by default.

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Using Physical CD/DVD

To use a physical CD-ROM or DVD: 
# For CD-ROM
qemu-system-x86_64 -m 2048 -hda myvm.img -cdrom /dev/cdrom -boot d

# For DVD
qemu-system-x86_64 -m 2048 -hda myvm.img -cdrom /dev/dvd -boot d

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Enabling KVM Acceleration

If you installed KVM, enable it for better performance: 
qemu-system-x86_64 -enable-kvm -m 2048 -hda debian9.img
KVM acceleration can improve VM performance by 10-20x compared to pure emulation.

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Common QEMU Options

Memory and CPU
-m 2048              # Allocate 2 GB RAM
-smp 2               # Use 2 CPU cores
-enable-kvm          # Enable KVM acceleration
Storage
-hda disk.img        # Primary hard disk
-hdb disk2.img       # Secondary hard disk
-cdrom file.iso      # CD-ROM drive
Networking
-net nic             # Add a network interface card
-net user            # Use user-mode networking (NAT)
Display
-vga std             # Standard VGA
-vga qxl             # QXL paravirtual graphics
-display sdl         # Use SDL display
-display gtk         # Use GTK display
Boot Options
-boot d              # Boot from CD-ROM
-boot c              # Boot from hard disk (default)

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Disk Image Management

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Check image information:

qemu-img info debian9.img

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Convert image formats:

# Convert VDI to qcow2
qemu-img convert -f vdi -O qcow2 source.vdi destination.qcow2

# Convert VMDK to qcow2
qemu-img convert -f vmdk -O qcow2 source.vmdk destination.qcow2

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Resize disk:

qemu-img resize debian9.img +5G

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Networking Configuration

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User-mode networking (NAT):

qemu-system-x86_64 -m 2048 -hda myvm.img -net nic -net user

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Bridge networking:

qemu-system-x86_64 -m 2048 -hda myvm.img -net nic -net bridge,br=virbr0
Bridge networking requires additional configuration on the host system and may need root privileges.

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Next Steps

After mastering basic QEMU usage:
  • libvirt: Use virt-manager for graphical VM management
  • Snapshots: Take VM snapshots with qcow2 format
  • Networking: Configure advanced network topologies
  • Performance: Optimize VMs with virtio drivers
  • Cloud Images: Use cloud-init images for automation

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Troubleshooting

VM is very slow
  • Ensure KVM is installed and -enable-kvm is used
  • Check that virtualization is enabled in BIOS
  • Allocate more RAM with -m
  • Try -cpu host for better CPU passthrough
No display window appears
  • Try different display options: -display sdl or -display gtk
  • Check that X11 is running if on Linux desktop
  • Use -nographic for serial console only
Network not working
  • Verify -net nic -net user is specified
  • Check firewall settings on host
  • Try different network modes

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Additional Resources

This tutorial is based on content from Fortinux

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