Don't usually post advice but against a 91/74 villian you messed this hand up pre and on the flop and put yourself in a bad spot.
pre - Bet more. i would say make it 8 or 8.5 BB. you are expecting a call. build the pot up. don't bet 6 "hoping to keep him in the hand". by betting 6, his calling range, which is already wide, is basically 100% of his hands.
flop - not a great flop for kk. bet more. again, he will probably call with any piece or any draw. bet big and make him pay. as played, more like 10bb.
turn - great card. bet bigger!
river - horrible card. check and evaluate. i'm calling off most small bets but am folding to any large bets/shoves.
You can't value bet this river because think about it from the villians perspective. if he has AQ, he is almost never calling a value bet. what else are you hoping to get a call with on a value bet? if anything, if you think you have the best hand, this is a prime spot to check/call a bluff. but again, you're probably beat at this point.