I was able to follow the instruction on http://wiki.xenproject.org/wiki/Install_Xen_4.2.1_with_Remus_and_DRBD_on_Ubuntu_12.10  to get a working Remus with PV guests. 

But now I am trying to do the same thing but for HVM guests. So I created a HVM guest on Xen by following https://help.ubuntu.com/community/Xen with the disk location pointing to a logical volume I created name "ubuntu-hvm" under volume group name "gpu2"

After that I did "drbdadm create-md ubuntu-hvm"

I also edited the /etc/drbd.d/SystemHA_protoD.res so that the disk is pointing to the location where I install and setup the hvm guest. 

Now I have the drbd connetced between 2 servers. 

So I edit the .cfg file for the guest U from 

disk = ['phy:/dev/gpu2/ubuntu-hvm,hda,w','file:/root/ubuntu-12.04.5-server-amd64.iso,hdc:cdrom,r']

to 

but when I xm create the guest VM, it told me that "Boot from Hard Disk failed: could not read the boot disk" and short after the HVM guest U is shutdown. 

Please let me know if you need anymore details. Thank you!

Victor