I allow myself to ask Wikipedia to get a solution in this tricky case:

"The plural form dwarfs has been traced to the 17th century. The alternative plural dwarves has been recorded in the early 18th century, but was not generally accepted until used by philologist J. R. R. Tolkien in his fantasy novel The Hobbit. Neither spelling represents the regular phonetic development of the Old English plural dweorgas, namely dwarrows; rather, they descend from a new plural formed in Middle English from the singular stem. Similarly, the old inherited plural dwarrows acquired a singular dwarrow.[1] Although dwarrow has passed from the language, both dwarfs and dwarves are in current use. Many grammarians prefer dwarfs, many fantasists prefer dwarves. The form dwarfs is generally used for real people affected by dwarfism; the form dwarves is used for the mythical people described by Tolkien and others."