Windows Server 2008 (R2) blog by Kurt Roggen [BE]

Hyper-V provides virtual machines with a pool of Media Access Control (MAC) addresses when Hyper-V is installed. Hyper-V has an algorithm to avoid duplicate MAC addresses on a single host, but not across multiple hosts. A crucial element in this algorithm is the Hyper-V host IPv4                                                                                                                              address (last 2 octets).

This article describes how MAC address pools work in Hyper-V, how it is possible to accidentally deploy hosts with duplicate MAC address pools, how to modify the default MAC pool and how using SCVMM 2008 to manage your Hyper-V hosts can prevent duplicate MAC addresses. 

The table below shows how to identify VM created with Hyper-V or VMM based in the MAC Address identifier.

NOTE: Since Windows Server 2008 R2 Hyper-V, you can change this MAC address pool from the Hyper-V Manager console instead of going through registry keys (MinimumMacAddress/MaximumMacAddress).

Learn more on how Hyper-V uses a MAC pool per Hyper-V host  and how VMM uses a MAC address pool for all VMHosts (Hyper-V, Virtual Server, VMware ESX).
For more information: VirtualizationAdmin.com: Dealing with MAC addresses in Hyper-V and VMM