What decreases kinetic energy?

Kinetic energy is the energy that an object has when it is in motion, so anything that will stop the motion of an object will decrease it's kinetic energy. The motion of an object can be stopped by any outside force acting on the object, whether sudden (think of stopping a rolling soccer ball with your foot -- that's decreasing it's kinetic energy), or continuous (the friction of that ball against the air and ground will eventually cause it to stop rolling on its own).

In the vacuum of space where there is no air to create friction a projectile will continue moving at the same speed without stopping and without decreasing its kinetic energy.