I'd pick Kovalev to stop Stevenson in 4 rounds.
The way I see it is, Stevenson has power but has been KO'd by a journeyman with marshmallow hands (8 KO's in 44 fights)
Kovalev has power and also has a granite chin.
I believe Kovalev has better footwork and movement too.
This taken into consideration, Kovalev wins.
Kovalev TKO RD 7, both guys get hurt.
This would be a good fight for sure.