You'll still get flats, but at a rate about 10% those without kevlar. It works for thorns, small pieces of glass and many metal pieces. A long piece of glass, screw, or funky metal debris may still get thru. Schwalbe marathons and specialized armadillos have the best reps for flat protection. Marathon Plus will protect against tacks and should be slightly lighter than your MTB tires. Most riders on forum with schwalbe tires ride with marathons to save a bit of weight. They even have a slick version. Some riders say they have riden thousands of miles without a flat.
You could also add a mr tuffy liner. some swear by it, but I think it's unnecessary.