Qemu KVM libvirt Manual (10): KVM restrictions

Overcommits

• KVM allows for both memory and disk space overcommit.
• However, hard errors resulting from exceeding available resources will result in guest failures.
• CPU overcommit is also supported but carries performance implications.

Time Synchronization

• kvm-clock 
• NTP or similar network based time keeping protocols are also highly recommended (for VM host server and Vm guest) to help maintain a stable time.
• Running NTP inside the guest is not recommended when usingkvm-clock.

MAC addresses

• If no MAC address is specified for a nic, a default MAC address will be assigned.
• This may result in network problems when more than one Nic hosts es the same MAC address.
• It is recommended to always assure a unique MAC address has been assigned for each Nic.

Live migration

• Live migration is only possible between Vm host servers with the same CPU features and no physical devices passed from host to guest.
• Guest storage has to be accessible from both VM host servers and guest definitions need to be compatible.
• VM host server and Vm guests need to have proper timekeeping installed.

User Permissions

• The management tools (Virtual Machine manager,Virsh,VM-install) Need to authenticatelibvirt

Suspending/hibernating the VM host server

• Suspending or hibernating the VM host server system while guests are running is not supported.

Performance