There is a trope common among many fantasy stories that Elves have a special power called Glamour, which is a kind of magical make-up.
In the Sandman comics, an Elf girl is given to the Dream Lord, and he tells her that she doesn't need to use her Glamour in the land of Dream; she suddenly becomes much shorter, older, and uglier. I seem to recall something similar in Spencer's The Fairy Queen (the distinction between Elves and Fairies has not always been so clear, cf. Fairies by Brian Froud).
Under this interpretation, the scars are real, and Thranduil's normal fair appearance is an enchantment or spell that he casts over himself.