I've seen that people run VMs with KVM and QEMU Virtual CPUs but when I'm creating a VM using virt-manager it only show my real cpu cores not virtual CPUs. how can I activate QEMU Virtual CPU?
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When you configure a VM it gives you the option of assigning a number of CPU's. These -are- virtual CPU's. Generally the configuration presents these CPU's as copies of what the physical host contains but you do have the option of selecting a different CPU type, assuming that type is compatible with the physical hardware in the box.
I don't see why you should use the QEMU vCPU on a single host. It is really there for baseline compatibility for live migration between several different hosts. As for overcomitting, you can, but whether or not you should is a different question. It all depends on the actual loads the VMs are running.