I installed KVM on ubuntu 16.04 and when I run virt manager it shows this message:
Warning: KVM is not available. This may mean the KVM package is not installed, or the KVM kernel modules are not loaded. Your virtual machines may perform poorly
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You either haven't installed qemu-kvm or you don't have virtualization enabled from BIOS.
Run this to make sure that you have KVM modules installed:
sudo apt-get install kvm qemu
And run this to make sure you have virtualization enabled from BIOS (If you don't then enable that):
grep vmx /proc/cpuinfo