Hey Penny - Kumarakom is not a beach and is famous for backwaters. Varkala is a quiet beach frequented by young couples while Kovalam is a crowded beach and each has its own charms. Both these beaches have a nice mix of country tourists and foreigners.
Kovalam is busy and a very active beach- but I wouldn't say totally overcrowded. There are lots of shops and restaurants and cafe's to eat at and peruse . The beaches are swimmable
Varkala attracts a young crowd but it isn't a party place and I've seen plenty of young families there. There are plenty of hotels and restaurants on top of the cliff, you can eat fresh seafood and you can bargain always to get best prices. The beaches are rough here than Kovalam.
You can check out any of the 3 star and 4 star hotels rated at both these places and choose a nice place based on guest reviews.Personally, I loved Varkala because I prefer quite beaches :)