Theoretical XP distribution question

[Frug]

For ease of argument, let's assume 2 characters in a group, of different levels.

When they group and get party XP, is the XP distributed in such a way that they will:

* Level at the same rate? (eg: their level difference will never change; if they were 2 levels apart when they started, they will forever remain so)
* converge? (eg: the lower level char will gain levels at a faster rate until he matches the level of the higher level char)
* diverge? (eg: the higher level char will gain levels at a faster rate than the lower)
* Something else?

Is this relationship the same if the 2 chars have different xp penalties/curves, or is it invariant?