According to one programmer, Omar Dolaimy, in Stackoverflow forum:
I know GMail is not an Open-Source program so we can trace the code. But every website try to make the URL shorter so they ideally shouldn’t add redundant data to the URL, In the same time they doesn’t make any difference nor Error if they change.
But according to this comment of Mike Sego, who was Gmail Frontend Engineer, 2005-2007, full form of SHVA is Should Have Valid Authentication. It means SHVA characters appear in the URL only after successful valid authentication, and it makes sense.
[via Google Operating System]