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
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 qemu-kvm
And run this to make sure you have virtualization enabled from BIOS (If you don't then enable that):
grep vmx /proc/cpuinfo