How to convert Xen image to VirtualBox?
Use the VirtualBox command line utility - vboxmanage.
vboxmanage clonehd XEN_disk.img VIRTUALBOX_disk.vdi
or
vboxmanage internalcommands converthd XEN_disk.img VIRTUALBOX_disk.vdi
or
Cheat. Use clonezilla or any cloning method.
Boot to clonezilla on the xen virtual client and virtualbox client using a clonezilla iso. Clone from xen to virtualbox.