Please always read README files.

Always.

Do you want to fix this warning, otherwise qemu will be very slow.

Install kvm:

$ sudo apt install qemu-kvm

Make sure that you can access /dev/kvm . You can easily check with

$ file -s /dev/kvm

that you properly get "ERROR: cannot read `/dev/kvm' (Invalid argument)", which means that "file" was properly able to open kvm, just not smart enough to use it

If you get "cannot open /dev/kvm' (No such file or directory)", check in your BIOS setup to make sure that accelerated virtualization is activated, and make sure that thekvmintelorkvmamd` module is loaded.

If you get "no read permission", make sure you give your user permissions to use kvm:

$ sudo adduser $USER kvm

and log-out and log-in again.