It seems this package just doesn't exist:
apt-cache search kvm-source
#if it returns nothing it means the package doesn't exit
As long as we want to search only Ubuntu official repositories, we can also check on http://packages.ubuntu.com/. Here we see that the package kvm exists, but that kvm-source doesn't.
If you want to retrieve the source for this package, you should try
apt-get source kvm
(see man apt-get
for more info on the source
command)
Edit to answer comment:
You can check if a package is installed with e.g.:
dpkg --status kvm