I could not find any reference to this problem online - so I thought I would post the solution I finally found.

I was running an OpenSuse 11.1/XEN system running a SLES 11 virtual machine for the past few years. Planning an upgrade, I installed OpenSuse 13.1 on a new system, copied the VM image file and .xenconfig files from the old system to the new one and tried to boot the VM.

Upon running the "xl create" command, the system returned the error:
Parsing config from <filename>.xenconfig
Neither kernel nor bootloader specified

The old .xenconfig file read:
name="AAA1"
memory=512
disk=[ "tap:aio:/disk2a/<filename>.raw,xvda,w" ]
vif=[ "bridge=br0" ]
vfb = ["type=vnc,vncunused=1,vnclisten=0.0.0.0"]

Suspecting the newer version of Xen required some newer .xenconfig options, I copied the file /etc/xen/examples/xmexample1 configuration file and attempted to modify as needed. Stripping out the comments, the new xenconfig file read:

bootloader="pygrub"
memory=512
name="AAA1"
vif=[ "bridge=br0" ]
disk=[ '/xen/<filename>.raw,,xvda,w' ]
vfb = ["type=vnc,vncunused=1,vnclisten=0.0.0.0"]
root = "/dev/xvda"
extra = ""

After issuing the "xl create" command the system would start booting and crash with the following message:
[ 0.903049] EXT3-fs (xvda): error: can't find ext3 filesystem on dev xvda.

After much fruitless searching and playing around, I found the problem was the "root = " line. If I commented out that line, the VM booted correctly.

Not sure what is going on here, but the fix seems to work.