Aprogas has put together a very useful howto showing how you can create a FreeBSD Xen PV domU.

Xen is virtualization software, which lets you run multiple operating systems on the same physical machine. The goal of this guide is to show you how to create a FreeBSD 8 installation that will run as a guest OS, without the need for a processor that supports hardware-assisted virtualization. It would go beyond the scope of this guide to explain how exactly Xen works or what the differences between PV and HVM are, suffice to say dom0 is the host OS and domU the guest OS. Although FreeBSD fully supports being a Xen domU, it is not yet very well documented and not suited for novice Xen users. Especially the lack of a Xen-aware installer or pre-built Xen-aware images make things more difficult. If you are a Xen novice, I recommend starting first with pre-built Linux images, and then trying a NetBSD domU installation, before proceeding with FreeBSD.”

Continues (FreeBSD forums)