I agree with @Beanflicker - Hopkins takes punchers to school, like Pascal and Cloud, whereas he struggles with athletic boxers, like Calzaghe and Dawson. Kovalev doesn't fit into the Calzaghe/Dawson mold.

The only caveat I have on it is that Kovalev's power might actually disguise his boxing ability. I think he actually uses angles and spacing pretty well.

In any event, it's a fight that won't ever happen because of the HBO/Showtime feud.