You can use, "I am starving" when you are very hungry. :) The second sentence you mentioned, "I am starve", is not correct. Perhaps you meant, "I am starved." In which case the difference between 'starving' and 'starved' is tense. 'I am starving' implies that you are starving right now, while "I am starved' implies that you have already been starved. In other words, your starvation is complete. This is similar to the difference between "I am dying" and "I am dead."