What is better..OVZ or KVM...Well it depend,when it comes perfomance OVZ is winner and also has many other cool features and adventages,but if also has many disadventages expecually from end user point of view(user who consider to buy VPS),so it depend from whom point of view we are considering this and also for what is intended.
From provider/vendor perspective it is first choice for Linux virtualized infrastucture and mine first choice,in case when provider need Linux virtualized infrastucture for own needs OVZ is always first choice....you get only OVZ adventages and in same time main disadventages are excluded....since no one will resell to itself
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Now it is compltetly different story if look this from VPS provider perspective who will sell VPSs to diffirent users with different level of skills and different intention.First problem can ,and in most case they do, make VPS provider it self with overselling service more than 20% which is generally considered to be safe to go,but that is not only problem,it is one thing when provider/vendor need linux virtualized infrastucture for itself,but t is completly different story when you have 50 different users with totaly different needs, OVZ share kernel which mean it is not always possible to satisfy every one needs when it comes kernel level.
So,at the end it is all about your needs...both have adventages and disantventages