Memory under KVM?

Hi everyone, I'm new to the list and I have a couple questions that we are wondering about here at work...
we have notice that the KVM processes on the host take much more memory than the memory we have told the VM to use.. a ruff example.. if we tell KVM to use 2 gigs for one VM it will end up showing on the host process list for that VM like 3 gigs or more...
Why do I ask this? well we need to figure out how much memory to add to our host server so we can calculate the number of VM's we can run there etc etc..

Thanks for the help

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