Kovalev probably stops Hearns. I don't like Hearns's chances against a powerful natural LHW with pretty solid skills. Hearns peaked 3-4 divisions below.
Hearns chin wouldn't last in the distance in this fight. KO Kovalev probably in the first half of the fight.
Hearns wins at LHW are better than Kovalev's and he could outbox Kovalev and maybe KO him. Kovalev could get the KO if he presses but if he eats bombs on the way in he could be ko'd himself.
Hearns is a great fighter...even at light heavy,four divisions up from his prime weight, he carried some power. If Kovalev got KD against Boone could Hearns do the same? Sergey would need to give Hearns some respect...but with that said, I think the naturally bigger man would win in the middle rounds.