How to figure total voltage with two voltage sources in the circuit
The reason you'd subtract is because the positive terminals are facing each other. You could compare it to forces applied to an object in physics. If you have a box and one person is pushing from each side and person A is pushing with 48 Newtons of force from one side and person B is pushing with 30 Newtons of force from the other side, the object will move away from person A as if it was being pushed with 18 Newtons of force. If the two voltage sources both had their terminals in the same direction, then it would be 48+30 = 78 volts. 

The second part of your question about KVL; you need math to figure out the current but yes, the current is the same around the loop.