The expression itself, "hills like white elephants," is actually a simile, not a metaphor, since it uses "like." White elephants are used to refer to something unwanted or undesired. In this context, they are also symbolic in the story of her undesired pregnancy. The story also develops the relationship between the man and the woman. She isn't convinced that she wants to have the abortion, and he wants her to have it, and pressures her to have it, all the while he says that the choice is hers.
Because this title contains the word "like," it is technically a simile. Any comparison using like or as qualifies as a simile. On a deeper level, however, the "white elephants" within the title are actually symbols.
They represent anything that is not wanted, be that a pregnancy, a child, or something else entirely. Inasmuch as the white elephants represent an abstract concept, they qualify as symbols: one concrete thing standing for something less tangible.