Sometimes I need to convert a xen xva image to a vhd image for VirtualBox.
Here is a quick howto:

deadfish $ qemu-img convert -O raw input.xva output.raw
deadfish $ VBoxManage convertfromraw output.raw output.vhd --format VHD
Converting from raw image file="output.raw" to file="output.vhd"...
Creating dynamic image with size 688802304 bytes (657MB)...

And your done!