Giannis is a bit faster in the open court with longer strides than KD and has a lower body fat %, that's it.
He ain't more athletic than KD; he's not nearly as coordinated with his movements, doesn't have even 1/10th the hand eye coordination as far as shooting goes, and he isn't as stronger either; KD has been a legit 240 lbs for the past 4+ years while Giannis isn't even 225 lbs. People mistake his low body fat and defined build as "strength" when he's probably one of the weakest guys in the league for his size next to his teammate Thon; notice how all of the guys he's bullying in the paint this season are all guards and small wings? Notice how he has almost nothing resembling a post game; especially against legit frontcourt players? That's his lack of strength showing. Durant has one of the most expansive and diverse games in NBA history so his athletic gifts aren't shown off as much; a 7 footer who can pull up for a high % transition 3 (this is also an example of athleticism if you didn't know) doesn't need to barrel to the rim every play out of necessity like Giannis is doing this season.
KD better than Giannis and better than Bron. He gonna finish much higher in DPOY rankings and in first team all-defense over both of em for forwards too with Kawhi injured for such a large portion of the season. He been an underrated if not elite defender since 2014 if we're being honest; no one scored less on ISOs when guarded by any other player in the league than KD that season if I'm not mistaken.