"This tutorial provides step-by-step instructions on how to
install Xen (version 3.0.3) on a Debian Sarge (3.1) system.
"Xen lets you create guest operating systems (*nix operating
systems like Linux and FreeBSD), so called 'virtual machines' or
domUs, under a host operating system
(dom0). Using Xen you can separate your applications
into different virtual machines that are totally independent from
each other (e.g. a virtual machine for a mail server, a virtual
machine for a high-traffic web site, another virtual machine that
serves your customers' web sites, a virtual machine for DNS, etc.),
but still use the same hardware..."