QEMU

QEMU (Quick EMUlator) is a generic and open source machine emulator and virtualizer. KVM (Kernel-based Virtual Machine) is the hardware-assisted virtualization infrastructure provided by the Linux kernel. It's using QEMU for operation system emulation.

BIOS
For KVM check, that your processor's hardware virtualization feature is enabled:

If not, enable "Intel VT", "AMD-V", etc. in your BIOS.

Kernel
You need to activate the following kernel options:

Software
Install :

Frontends
To make life easier, there are multiple user-friendly front-ends to QEMU:

Permissions
Add your user to the kvm group:

Usage

 * See QEMU options page for common options
 * Setup a Linux guest
 * Setup a Windows guest

External resources

 * KVM - Community Gentoo Wiki article