EVE Online and meatspace violence are examples of negative sum PvP. The stakes are what people bring into the competition, some of those stakes will be destroyed in the competition, and what the winner gains is equal to or less than what the loser lost. (Meatspace political lobbying is negative sum PvP for society as a whole.)
Tournament standings and poker are examples of zero sum PvP. There is a fixed pool of stakes, and your gain is exactly equal to someone else’s loss. Almost all status games (explicit or not) are zero sum, with status as a relative good such that one can only rise by displacing another. (The tournament itself can be a gain for competitors, but the fixed nature of the prize pools makes the competitive elements zero-sum. For me to get the first place prize, I must prevent someone else from getting it.) (Poker with a rake is negative sum for the players.)
Games are increasingly fond of positive sum PvP. Everyone fights, everyone gets a prize, everyone comes out ahead. In League of Legends, everyone gets influence (and winners get more). In Guild Wars 2 sPvP, there are no permanent costs, and everyone gains glory, rank progress, and achievement progress (and winners get more). (Meatspace economic competition is positive sum PvP for society as a whole, where winners are decided by producing greater value for less cost rather than by political lobbying.)
In all of these cases, we tend to discount or ignore the time spent. If you enjoy the game, spending time is not much of a cost, anyway. Time spent being entertained is the benefit, not the cost, although the time spent in-game almost certainly has a higher earnings potential than the cash value of the in-game benefit you gain, although you can potentially profit by poker. (In meatspace, the time spent may be the most important thing.)
Marvel Puzzle Quest tournaments use a mix of positive and zero sum systems. When you defeat an opponent, you gain points, usually more than they lose for losing. Those points add up to benefits (positive sum). There is also a ranked tournament structure with a fixed prize pool, where advancing necessarily displaces someone else (zero sum). Because you can spend in-game resources in the tournaments, the tourney competitions can become negative sum, although given rewards per win, you would need to be burning it fast, which can happen in the fight for first place.