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A quick look about KVM (http://kvm.qumranet.com/kvmwiki/KVM-Wiki):

KVM (for Kernel-based Virtual Machine) is a full virtualization solution for Linux on x86 hardware. It consists of a loadable kernel module (kvm.ko) and a userspace component.

Using KVM, one can run multiple virtual machines running unmodified Linux or Windows images. Each virtual machine has private virtualized hardware: a network card, disk, graphics adapter, etc.

The kernel component of KVM is included in mainline Linux, and will appear in Linux 2.6.20.

KVM is open source software.

So how to set it up under Debian? Since KVM is still under active development, using Debian sid (unstable) would be a good idea; on the other hand, a VT supported CPU is also required; finally, you should have X11 installed. You may handle the setup of KVM client in many different way: manual command line parameter, custom pre-defined start/stop/config shell scripting, or by using libvirt + virt-manager. For this simple mini-HOWTO, I will guide you though the 1st solution with the least dependence.

Prepare your Debian sid

First of all, let's setup your APT with unstable/testing/stable hybrid support. Here is my /etc/apt/sources.list as reference:

You may also setup /etc/apt/preference as below for additional APT-pinning handle (this example pin your Debian as unstable, check its priorities according to your needs):

Moreover, you will need to create /etc/apt/apt.conf.d/99cache as below, or else your cache may be busted:

Whatever you are now using Debian stable/testing, just run the following command and upgrade your Debian as unstable, install latest Linux kernel + kernal header + KVM + module-assistant, and install kqemu plug-in as spare:

After reboot, check your kernel version with uname -a, e.g.:

Setup your hardware and network

Let's probe KVM required modules:

And check the result with lsmod

Next we should setup the network connection. Before further configuration, stop your current active network interface:

Edit /etc/network/interface as like as below, e.g. replace the original eth0 configuration as br0, and add other required parameters:

Now start the new interface with:

Some kernel parameters are required, too. Add the following lines in /etc/sysctl.conf:

Load the new setting with:

For further more details of libvirt networking, please refer to here:
http://libvirt.org/formatnetwork.html#examplesNAT
http://wiki.libvirt.org/page/Networking

Reboot, and check your network with ifconfig.

Create KVM client disk image

I would like to suggest hold all your KVM client disk image under a specific directory, e.g.:

Go to that directory and run the following command (32GB image with qcow2 format):

Start your KVM client

Assume you hope to install Debian stable in this KVM client (prepare you debian-40r3-i386-netinst.iso in /srv), run the following command to start the image (you should run kvm --help for more detail of this configuration):

Then use any VNC client, and connect to this KVM client (e.g. localhost:5900). Start the installation and enjoy with it :)