What is XAPI?

A.
An extension of libvirt which adds live migration of virtual servers between arbitrary hypervisors.

B.
A ReSTful API which is used by xl to communicate with the Xen hypervisor.

C.
The communication interface between a paravirtualized Linux Kernel and the Xen hypervisor.

D.
A high level tool chain which provides advanced management tools and interfaces for Xen.

E.
The internal messaging system within the Xen hypervisor.