Paraphrased from the Aliens Adventure Game:
The Facehugger alien is hatched from an egg, and inserts a new egg into a host body, after which it dies. It's only purpose is finding and infecting hosts.
The Chestburster, the immature form of an Alien warrior, requires a large supploy of food, inorganiuc material and water. This totals twice its eventual adult weight, which is 300 lbs, of which 1/4 must be hydrocarbon based, 1/2 inorganics, especially those high in silicon, and 1/4 water. After it reaches adult form, it doesn't require food any longer, but it can lay an egg a day on consumption of 15 lbs of organic and inorganic material. Presumably, food would also be required to resupply its web and stingers.