The following points provide a brief overview of KVM as it pertains to system performance, as well as process and thread management:
When using KVM, guests run as a Linux processes on the host.
Virtual CPUs (vCPUs) are implemented as normal threads, handled by the Linux scheduler.
Guests do not automatically inherit features such as NUMA and huge pages from the kernel.
Disk and network I/O settings in the host have a significant performance impact.
Network traffic typically travels through a software-based bridge.
Depending on the devices and their models, there might be significant overhead due to emulation of that particular hardware.