Update (20131106): Please see FreeBSD 10 On CloudStack + XenServer Blog post instead.
UPDATE (20131015): Major changes have gone into FreeBSD 10 including fixes for the xenbusb_nop_confighook_cb. BETA2 onwards would support XenServer out of the box. The ISOs are available at http://ftp.freebsd.org/pub/FreeBSD/releases/ISO-IMAGES/10.0/.
Pretty excited to have FreeBSD 10.0-CURRENT running under XenServer 6.1 in HVM mode using the XENHVM configs.
Follow the instructions in http://home.uncon.net/dokuwiki/doku.php?id=xenserver:freebsd to workaround the ‘xenbusb_nop_confighook_cb’ issue on boot-up. UPDATE (20131015):This is now fixed with r256423