Here's my setup:
SLES 10 SP4 64-bit XEN Host
I have TWO physical disk-backed DomU
One is paravirtualized SLES 10 SP3 64-bit
One is Fully Virtualized SLES 11 SP1 32-bit

Rather than upgrade the host via offline/inplace upgrade to SLES11 SP2, I powered off the server.
Disconnected the LUNs via our SAN (we boot from SAN)
Created a new LUN0
Attached it to the server
Booted up the SLES11 SP2 code and installed SLES11 SP2 as Physical machine
After install, I patched server
Then I went into Yast and added on the XEN Hypervisor and Tools
All is good.

Now, I powered server off
I re-attached my two other LUNs that hold my DomU
I boot up server
Server can see LUN's just fine

I manually went into virtual manager and created a paravirtualized VM with the same settings as what was on the SLES10 host (I have the xm -l file I exported so I know what the config was). I point it to the same physical disk (/dev/disk/by-id/scsi-bighairyGUID)

However, when the DomU tries to boot all I get is:
Boot failed
Boot loader didn't return any data


????

The FULLY virtual machine actually boots, but for some reason cannot find the / partion (but obviously finds the /boot partition).

Now, if I power off the server, remove the SLES11 SP2 Dom0 boot lun, re-attach the SLES10 SP4 Dom0 boot lun, boot it up, things load just fine.

Is there some conversion that needs to be done between a DomU that was originally created in SLES10 SP4 64-bit when going to SLES11 SP2 64-bit?
I couldn't find anything in the SLES 11 SP2 docs EXCEPT if you created a VM in SLES10 (no SP), but that is not the case here.