It depends on what you want to say. "to be starving" - If you are very hungry then you may say "I am starving". This is called "hyperbole". You are exaggerating because you are not actually starving, you're just saying this to show that you are extremely hungry. If you are literally starving then it means you are not eating anything and may "die of starvation". "being starved" - If someone is "being starved" then it is usually meant in a literal sense. It typically means that someone is not letting another person or animal eat, so they are "being starved (to death)".

You likely want "being starved" As in, "The prisoners are being starved to death."