Before agreeing to Mr. Timothy's post, I forgot to brush up what is really meant by the XSD type xs:string. If we look at xs:string definition at, https://www.w3.org/TR/xmlschema-2/#string it says

"The string datatype represents character strings in XML. The ·value space· of string is the set of finite-length sequences of characters (as defined in [XML 1.0 (Second Edition)]) that ·match· the Char production from [XML 1.0 (Second Edition)]."

While writing +1, I suspected that XSD's xs:string mirrors Java's String data type. But actually, xs:string takes its value space from XML 1.0 (Second Edition).