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.