If you have an AMD CPU with SVM extensions, then you can create a fully-virtualised HVM and install FreeBSD as per normal into it. I had 32-bit and 64-bit FreeBSD 6.3 and 7.0 (4 VMs) installed on a 64-bit version of Xen 3.2.0 on Debian for a while.
There are issues with doing the same on Intel CPUs with VMX. For some people it works, for others it doesn't.
Don't bother trying to get FreeBSD to run in a paravirtualised setup.
A much smoother, easier, more understandle method is to use [
http://kvm.qumranet.com/kvmwiki KVM] with any distro that has a 2.6.24 kernel. We had a test setup with 2x 32-bit FreeBSD 6.3 doing buildworlds, 2x 64-bit FreeBSD 7.0 doing portsnaps, 64-bit Debian Etch doing dist-upgrade, and a 32-bit Windows XP doing a SP2 install, all simultaneously, without slowing any of them down noticeably. (2x dual-core Opteron 2 GHz, 8 GB RAM, 4 TB of disk in RAID6)