PostgreSQL does not have in-place update in the first place, on every UPDATE it does a DELETE+INSERT. So "partial update" does not make much sense, because you'll create a new copy of the row anyway.
In practice it's more complicated of course. A nice intro to MVCC is available here: http://momjian.us/main/writings/pgsql/mvcc.pdf (and in the Bruce's other talks http://momjian.us/main/presentations/internals.html).
Or maybe you mean something else by Multi-value-Concurrency. In that case please elaborate.