> I would like to know the difference between "n_tup_upd" and > "n_tup_hot_upd".
A HOT update is used when none of the updated columns are used in an index and there is room for the new tuple (version of the row) on the same page as the old tuple. This is faster for a number of reasons, and cleanup of the old tuple is a little different.
If you want the gory implementation details, take a look at this file in the source tree: