The XML data type is not intended to be comma delimited, it is a different way of structuring data. So you are not able to pass the whole xml data as a string? I have no experience with passing a xml structure. Does your xml data originate from Sql Server? One thing you could try is setting your variable as a OBJECT type and pass it the results from a xml formatted recordset.

Example :

SELECTCOl1, COl2, ... , Coln

FROM MyTable

FOR XML PATH ('YourRecords'), ELEMENTS

If this does not work you may need to add another method to the web service to accept a string (tags and all) and cast it into XML on the backend.