The .img file, from ec2-unbundle, is the file that typically uses extension .raw on some virtualization platforms - It's just the raw disk contents byte-for-byte.
Most virtualization software can use this .raw file as the disk - I haven't done much with XEN, but VirtualBox and QEMU will both use it fine - HyperV and Virtual Server will need it converted to VHD format. I would expect XEN to use it.
A quick google for xen and raw shows this may be the command (but a lot of useless posts came back so not sure):
disk = [ 'file:/home/xen/ec2image/ec2file.img,hda,w' ]
NOTE: You will likely have driver issues, you didn't say if this is windows, linux or other. It will likely detect new hardware.
LICENSES: If this is Windows you are going to have license issues (perhaps not technical but definately legally speaking).