Oracle VM Server is based on Xen, so also Oracle VM Server use this name.
Dom0: Management domain
Most of the responsibility of hardware detection in an Oracle VM Server environment is passed to the
management domain, referred to as domain zero (or dom0). On x86-based servers, the dom0 kernel is
actually a small-footprint Linux kernel with support for a broad array of devices, file systems, and software
RAID and volume management. In Oracle VM Server, the dom0 is tasked with providing access to much of
the system hardware, creating, destroying and controlling guest operating systems, and presenting those
guests with a set of common virtual hardware.
DomU: Guest operating systems each have their own management domain called a "user domain", abbreviated to "domU". These domains are unprivileged domains with no direct access to the hardware or device drivers. Each domU is started alongside dom0 running on Oracle VM Server.
I hope this can help you