I have seen Ovechkin quite a bit via video-clips & international games that I get to see now and then.
Ovechkin, I'd compare kind of to a Joe Sakic on the wing. A young Joe Sakic especially. You can tell there is a difference though, and most of that comes from Russian coaches compared to Canadian coaches. One thing that really strikes me about both of them is they love to shoot the puck, which is great.
Malkin I know a little less about, but I have seen him play. I agree with another poster than Nash is not a good comparison. From what I've seen, a playmaking Alexei Yashin would be more apt. I don't think Malkin will ever be a power forward in the NHL, but he will use his big frame to fend off attackers. He doesn't have a shot like Yashin did, and maybe he's not quite as talented as Yashin was. He does seem to have more heart though.
In terms of character, Malkin from what I know of him has heart, but that's about all I know about him character-wise. Ovechkin I know quite a bit more about, and let me tell you, he is for the most part a very quiet, humble person off the ice, much like Joe Sakic. He is not scared to say when he played bad, and I don't think you'll ever see him talk about how good he is either. Ovechkin may lack some of the 'fire' that Kovalchuk has, but Sakic lacked a bit of the fire Yzerman had, and it didn't hold him back.