Did you try with xen tools :
xen-create-image --hostname=xen1.example.com --size=2Gb --swap=256Mb --ide --ip=192.168.2.123 --netmask=255.255.255.0 --gateway=192.168.2.1 --force --dir=/home/xen --memory=256Mb --arch=amd64 --kernel=/boot/vmlinuz-2.6.24-19-xen --initrd=/boot/initrd.img-2.6.24-19-xen --install-method=debootstrap --dist=hardy --mirror=http://archive.ubuntu.com/ubuntu/ --passwd
And after that you need to replace "file:" with "tap:aio" in the cfg file.
This will create you an image file but you can mount it and copy the content to a partition afterward. You simply need to change the line "file:/home/xen.../disk.img" with something like "phy:/dev/md3,hda1,w"
Hope it helps...
More details :
http://blogama.org/node/9