I believe envy is a positive thing. I'm not sure jealousy is a positive thing though, it strikes me as inherently negative. The difference between the two of these would be that jealousy is a fear of loss while envy is an admiration for what someone else possesses, whether skill or spouse or wealth.
That's not to say envy should be sought and jealousy avoided -- some situations call more for one or another, and as I understand it jealousy is a reflexive human emotion. We grapple with it and aren't often rational when it arises. The biggest difference for me is whether one is appropriate or not. Should someone flirt with my wife and get her to smile, it's possible I would feel jealous (appropriately). Regardless of that, it wouldn't be right in any situation to allow that emotion to rule me/her/them, we're ideally more civilized than that.
Many of the people I've met who identify themselves as 'jealous' seem to use it to justify an array of angry or controlling responses, instead of simply as a personal quirk. I might be unfairly biased
