Discuss XSD here:
Exactly where is that .xsd?
Here are possible sources of a workable OSM .xsd
Best found so far
oschrenk Feb 4, 2013 https://github.com/oschrenk/osm/blob/master/osm-io/src/main/resources/OSMSchema.xsd
Note: there is no official .xsd See: https://help.openstreetmap.org/questions/41078/location-and-use-of-xsd-schema-files See also OSM XML#OSM XML file format notes. The above .xsd's may not exactly match the XML file you recieve. OSM does NOT use XML at its core. The core data at OSM is xml tagged by the API.
Older Forum topic From http://forum.openstreetmap.org/viewtopic.php?id=7186
Older Help topic (note 3,500 views!) https://help.openstreetmap.org/questions/8682/xml-schema-for-nominatim-responses