I would pronounce it Shoh-ahn. I had a relative with a similar name, Joan. He was Galician, and I think Xoán is the Galician form of John. So, not only would I pronounce it Shoh-ahn but it is correctly pronounced Shoh-ahn. =D I think others would totally massacre it though.
"I would rather live my life as if there is a God and die to find out there isn't, than live my life as if there isn't and die to find out there is." -Albert Camus