All versions of pfSense are compatible with Xen 3.2.1 with HVM. Paravirtualization is not supported in current stable FreeBSD releases so it is not possible at this time, but HVM does work properly with the real mode boot fixes added in 3.2.1. You will need to use the make option “vmxassist=n”. To our knowledge, the Xen packages included with most major Linux distributions do not do this at this time, so you must compile it yourself with this option.
Thanks to Brian Zushi for this information. He restored his pfSense configuration from a physical box into a Xen VM and is currently running it in production.