I found these steps at http://unixfoo.blogspot.com/2008/12/vmware-to-xen-conversion.html. I found them pretty helpful, but I have modified them a little to show what I had to do.
If this is a Windows VM, you must first modify the storage controller to make it more “universal” and allow the controller change. You can follow the directions in the Windows Support KB Article (http://support.microsoft.com/kb/314082/) or you can do what I did. Go to Device Manager and double-click the storage driver (ide controller, raid controller, scsi…whatever your machine has) and choose update driver. In the list of updated drivers, choose from a list and choose the Microsoft Standard Controller. Click OK, and then shutdown your computer and follow the directions below.
1) Delete any old Snapshots. This will get rid of the duplicate files and merge the changes back into one vmdk file.
2) Halt the vmware VM . Check the format of the vmware vmdk file
[root@mtdhost]# qemu-img info mtdhost-xp.vmdk
file format: raw
virtual size: 15G
disk size: 15G
3) If the file format is “raw”, we need not convert the image file using qemu-img convert, just rename to .img file. If it is not “raw”, use “qemu-img” to convert the image to raw format. Otherwise skip step 4.
4) How to convert using qemu-img.
[root@mtdhost]# qemu-img convert mtdhost-xp.vmdk -O raw /VM/mtdhost-xp.img
5) Create xen config file in /etc/xen/mtdhost-xp (or use the virt-manager utility and choose the newly converted file as the harddrive image), with the renamed/converted image file as disk.
disk = [ ‘file:/VM/mtdhost-xp.img,hda,w’ ]
6) Boot up the xen domU guest. It should boot up fine.