Centos installs xen 3.0.1 which is old as hell, and 3.4 is way better. Also Freebsd will not install inside of 3.0.1 without a lot of extra work. So easy work around is to install this repo in yum.
http://www.gitco.de/repo/
$ cd /etc/yum.repos.d
$ wget http://www.gitco.de/repo/CentOS5-GITCO_x86_64.repo
$ sudo yum groupremove Virtualization
$ sudo yum groupinstall Virtualization
$ sudo reboot
[root@xen1 xen]# cat freebsd.hvm
name = "FreeBSD7"
builder = "hvm"
memory = "1024"
disk = ['file:/d2/images/bsdsmtpgateway/disk1.img,hda,w','file:/d2/iso/7.2-RELEASE-i386-dvd1.iso,hdc:cdrom,r']
vif = [ "mac=00:16:3e:70:66:ee,bridge=xenbr0" ]
device_model = "/usr/lib64/xen/bin/qemu-dm"
kernel = "/usr/lib/xen/boot/hvmloader"
vnc=1
boot="d"
vcpus=1
acpi="0"
pae="0"
serial = "pty" # enable serial console
on_reboot = 'restart'
on_crash = 'restart'
$ xm create freebsd.hvm
$ sudo lsof -i -n -P | grep qemu #find out what port vnc is listening on.
qemu-dm 6045 root 14u IPv4 18110 TCP 127.0.0.1:5900 (LISTEN)
From remote box;
$ ssh -L 5900:localhost:5900 hostname
$ vncviewer localhost
