Sliver creatures you control get +2/+0.
Illustrated by David Gaillet · Jumpstart: Historic Horizons · #430
There are no rulings for Cleaving Sliver.
Sliver creatures you control have haste. (They can attack and as soon as they come under your control.)
Creatures you control get +1/+0.
Sliver creatures you control have menace. (They can't be blocked except by two or more creatures.)
All Sliver creatures get +2/+0.
Green creatures you control get +0/+2.