They both have that pronounced elbow bend on their forehand finishes which makes their forehands look similar. That said Medvedev has an even uglier FH which is saying a lot. Rest of their games aren't similar.
The greatest similarity is their defensive-backhand prowess.
But Djokovic has the greater forehand, while Med has the greater serve.
Djokovic is a lot more often closer to the baseline, while Med varies his position frequently.