Commenter KathosAnnora is correct about the two pronunciations of this name. When I lived in a Welsh-speaking part of Wales my next-door neighbours explained this difference in the name's pronunciation to me, one being from South Wales and one from the North.
Actually I believe this name is pronounced "HAYL-wen". At least, that is how I hear it pronounced in North Wales (perhaps "HIEL-wen" is the pronunciation in South Wales?)It's a nice name nonetheless, with a very pretty meaning.