Windows Virtual PC is the latest Microsoft virtualization technology designed for Windows 7. It is the runtime engine for Windows XP Mode to provide a virtual Windows environment for Windows 7. Windows XP Mode, a new benefit of Windows 7 Professional and Windows 7 Ultimate, provides a virtual Windows XP SP3 runtime environment within Windows 7, and allows one to install and run Windows XP specific application directly from your Windows 7-based PC. Notice Windows XP Mode is 32-bit only while Windows 7 has both 32-bit and 64-bit versions. Windows XP Mode is a full virtual version of Windows XP SP3 and follows the same support lifecycle as Windows XP whose support phase ends in 2014.
On the other hand, if running Windows Vista or Windows XP on a physical machine, one can use Microsoft Virtual PC 2007 to run multiple operating systems at the same time on the same physical computer. There are noticeable differences between Windows Virtual PC and Microsoft Virtual PC. For instance the former requires hardware assisted-virtualization, supports USB, and is integrated with Windows XP Mode, while the latter does not. Here is the information to Compare some of the many features of Virtual PC 2007 to Windows Virtual PC.