@mayankcpdixit gave the answer up there in a response to the OP’s original question, but I’ll put it here again in case anyone misses it.
You do not need to uninstall everything, and if I had to manually add a new path link for every npm package I try to install I’d probably shoot myself.
npm install -g karma-cli
Boom. Now you have karma command lines installed. Just like Grunt.
Edit: Please don’t forget to upvote @mayankcpdixit as well, he commented directly on the original post, but didn’t actually “answer” the question.