When a guy says you are sweet and kind it means just that ... you have a good personality; are kind and considerate and it's a great compliment to you. Sounds like this guy is really interested in you because he has picked out your good points in your personality.
When a guy says you're such a sweetheart, he is likely expressing that he finds you kind, caring, and thoughtful. It's a compliment that highlights your positive and considerate qualities.