Evan could be... Another form of John. John in Spanish is Juan. The j resembles an I, and the U resembles a V. So Russians made Ivan. Badly pronounced, it could make Evan. Evanna could be its feminine form. Evanna Lynch, the actress, plays Luna Lovegood in Harry Potter, and she is so pretty!
From the Hebrew, Eben or Even, meaning a rock or a stone. It is found in the compound word Ebenezer in 1 Samuel 7:12 saying "Then Samuel took a stone and set it up between Mizpah and Shen. He named it Ebenezer, saying, "Thus far the LORD has been our help."