Apache by default is configured to serve html files, if you want to setup Apache to serve other files then you have to edit the "httpd.conf" file. This file is usually located in your apache installation folder conf directory, open the file using notepad search for the following tag "<IfModule mime_module>" within this tag you have to tell apache to serve other content types, you have to add the new type that you want to publish with the following values
AddType application/xhtml+xml .xhtml .xml
AddEncoding xhtml xml
The first variable defines the type of media and the second tell the encoding that you want to use, this will instruct the browser to handle these media types respectively. For a list of registered media types see here.