That proves absolutely nothing.
Plus minus measures productivity, not defensive acumen. It has a ton of noise since +/- is affected by teammates, quality of competition, and goaltending (this variable should not matter much over a large enough sample, although a season's worth isn't big enough in my opinion). Kovalchuk and Hossa did not have identical linemates and opponents, so you're argument is useless. In addition Triumph already stated that Hossa's presence on the penalty kill would artificially inflate his +/- compared to Kovalchuk's. Finally, wingers do not have a tremendous effect on preventing goals from being scored. That responsibility lies with defensemen and centers, so don't immediately suspect that differences in +/- among two wingers will be because of their defensive ability.
And another thing. Hossa is known as an extremely good defensive player, so it's logical that he would have a higher +/- than Kovalchuk (ignoring all the points above). The fact that he has a lower +/- doesn't automatically characterize Kovalchuk as bad defensively, or a floater. Most Atlanta fans recognize the fact that he has matured as a hockey player over the last several years and is fairly responsible in all facets of the game. You're touting a stereotype from 2004.