I'd say both are correct because I've heard both variants but one implies that the action is already over, hence the use of past tense (loved). I would use a construction like the second sentence if the action is still going on (in this case if the person still loves you). As far as I know though, the first one is more commonly used and the tenses match (both in past tense) so I assume that grammatically speaking the first one is the correct one.

Correct form: I didn't know you loved me that much. It is a sentence in the past-tense form so 'loved' needs to be used instead of 'love'.