Once pfsense is running, drop into a console as root (or SSH in) and run the following. Then reboot!
pkg install xe-guest-utilities
echo 'xenguest_enable="YES"' >> /etc/rc.conf.local
ln -s /usr/local/etc/rc.d/xenguest /usr/local/etc/rc.d/xenguest.sh
service xenguest start
Thanks to Tom for this bit of info.