This guide explains how you can set up fully-virtualized guests (HVM) with Xen 3.2 on a Debian Lenny x86_64 host system. HVM stands for HardwareVirtualMachine; to set up such guests, you need a CPU that supports hardware virtualization (Intel VT or AMD-V). Hardware virtualization allows you to install unmodified guest systems (in contrast to paravirtualization where the guest kernel needs to be modified); that way you cannot only virtualize OpenSource operating systems like Linux and BSD, but also closed-source operating systems like Windows where you cannot modify the kernel.