As plurals of elf, both elfs and elves have long histories in English, and there is plenty of precedent for both, but elves has always been the more common form by far, and it remains so to this day. In 21st-century edited writing, it appears many times more often than elfs. The latter form, moreover, is mostly shunned by English reference books and doesn’t pass spell check, for what that’s worth.
Elf differs from dwarf, which has two plurals—one for the real world (dwarfs) and one for fantasy worlds (dwarves). The difference in these plurals is useful because it distinguishes a varied collection of fantasy creatures from a varied group of real people, some of whom perhaps wouldn’t wish to be associated with the fantasy creatures. The case of elf is different because there are no real-life elves, so we have no need for a down-to-earth form to distinguish real people from fantasy creatures.