Although Virtual Machine Monitor (VMM) producing companies like XenSource and VMware don't manufacture physical network adapters, they do provide virtual ones. Thus, they have their own OUIs.

In fact, VMware has two OUIs. "00-0c-29" for automatically generated MAC addresses, and "00-50-56" for MAC addresses that are manually set.

For VMware dynamic MAC address, the first 3 fields are 00:0c:29, and the remaining 3 fields are random.

For VMware static MAC, the first 3 fields are 00:50:56, and the remaining 3 fields are random, with the first bit of the first random field set 0.

XenSource has OUI "00-16-3e". For Xen guests MAC address, the first 3 fields are "00:16:3e", and the remaining 3 fields are random, with the first bit of the first random field set 0.

You can use the shell script easymac.sh, which is derived from Easyvmx.com's original one, to generate for Xen and VMware guests.

You also can use the python tool easymac.py, which is derived from xend, to generate MAC address for Xen guests.

Reference

1. IEEE OUI and Company_id Assignments.
2. Easyvmx.com - Create virtual machines for VMware Player.