Well, you still have to consider that:) Why this works without you doing any extra legwork is because for you to run windows on Xen, unless your using the pv drivers, your Xen config exports an IDE disc into the vm. Unless you explicitly do *not* load an ide driver during install of the windows guest, it has this driver in the Critical Devices Database. It will just boot. You may have to make changes to the boot.ini file depending on the version of windows and format of the original vm but I doubt you need that.
Thanks - I was wondering what was going on under the lid
You can also convert the images as well (qemu-img convert -f vmdk... I thnk). If this image is running under ESX, then you should export the disc out with vmkfstools, http://www.vmts.net/vmkfstools.htm
Thats helpful too (qemu-img docs at http://bellard.org/qemu/qemu-doc.html#SEC19)
Thanks Stephen