MVCC provides a form of mutual exclusion among readers and writers on a single database object, while isolation is a more abstract guarantee that specifies the order in which transactions can read or write multiple objects. For this reason, database locking protocols, such as two-phase locking, are far more complex than locking disciplines used in concurrent programs. In general, this implies that using MVCC cannot eliminate the use of locking mechanisms in your database system.
From: Jose Faleiro's blog. (jmfaleiro.com)
