Oracle Text

Creatures you control are Slivers in addition to their other creature types.

Card Rulings

9/25/2006 If a creature is already a Sliver, Hivestone has no effect on it.
10/15/2006 Turns only your creatures on the battlefield, not in other zones, into Slivers. It won’t allow you to have Root Sliver on the battlefield and make your Grizzly Bears uncounterable, for example.