tonny - xvl is a bit... archaic. if you have a twisted mind, it's easy to get to grips with.
basically, it's a bit like php - you have a php.exe and you have a mod_php.
you can run php.exe as a stand-alone application (and you can run xvl as a stand-alone application) or to make it more useful you can use an apache module to talk to it.
xvl actually runs as a daemon (or more specifically, it runs an xvl script which says 'please listen on a unix domain socket!), which the apache module mod_xvl then talks to and hands over the HTTP request to it, for processing.
but like i said - it's _very_ archaic.
it _works_... and if you seriously want to inflict xml programming upon yourself, go for it.
but, unless you have a twisted mind, avoid xvl!