Elowen is a recent Cornish baby name meaning "elm." It may not take off in America like the last Cornish hit, Jennifer, but it's certainly pretty. She's part of a "linguistic revival," as Appellation Mountain puts it here. Her nickname could be Ellie, Elle or Ella, or a more unusual choice, Wendy. The pronunciation of Elowen is "ell-LOH-en," though most Americans will probably stick with "ELL-oh-wen." Please note that it is not spelled Elowyn. There is a similar name, Eowyn, but if you spell it Elowyn it will no longer be the Cornish name meaning elm tree, just some creative name.
Other great Cornish names include Penrose, Chesten (the Cornish form of Christine), Demelza, Denzel, Hammitt, Kerensa/Kerenza, Meraud (very similar to French gem name Emeraude, both meaning emerald, but Meraud is pronounced "meh-row"), Merryn, Morwenna, Tremain, Emblyn, Jory, Massen, Treeve and Cotton. Here's's a link to a post on Cornish names that includes Cornish saints and word names.