I found out more about them:

1) Windows 7 offers both Windows Virtual PC and Windows XP Mode.

2) Windows Virtual PC and Windows XP Mode are two different things.

3) To install Windows XP Mode, Windows Virtual PC will need to be installed too.

4) Windows Virtual PC on Win 7 is a Virtual PC.  A guest operating system

5) Windows XP Mode is to run a Win XP app side by side with a Win 7 app.  There is no guest operating system.

6) So again, Windows Virtual PC and Windows XP Mode are two different things.

7) Windows Virtual PC is provided in all versions of Win 7 (or most version, including Home Premium).

8) Windows XP Mode is only in Win 7 Professional, Enterprise, or Ultimate.  It is not in Home Premium version.