Kevlar is generally used in two different applications on tyres. It is commonly used for bead construction on folding (as opposed to wire bead) tyres, which has nothing to do with puncture protection, that simply makes tyres lighter as well as foldable.
The other purpose is of course puncture protection. Interestingly it is not necessarily the best puncture protection - Schwalbe e.g. use a thick layer of special rubber on their Marathon Plus, their top puncture protection tyre. Kevlar is lighter though.
You might be interested in the discussion
here.