What do xenomorphs eat?

This must seem like a very random question but jokes aside really what do xenomorphs eat? To me the appear carnivorous because lets see...they have sharp nails, fangs, a inner mouth(with sharp teeth), and a spear tail. However most every time they kill they never even nibble on the bodies. Although maybe they only eat organisms native to their home planet,if they have a home planet. The only time i ever seen one do anything to a human, was after the human was dead in Alien 3. Another theory i have (though i think it is unlikely) is that maybe some how they provide their own energy, maybe they soak it in their skin or something. thank you for your responses.