How do I get an XML response instead of a string using the Amazon MWS GetReport Reporting API operation?

You can set up your amazon seller account to generate XML reports as mentioned in this snippet from
 http://amazonpayments.s3.amazonaws.com/documents/Managing_Orders_with_MWS_APIs_en_US.pdf

2.3.2 Report Formats
Whether a seller receives reports in XML or flat-file format is 
determined by the seller account
configuration. The report type can be set in Seller Central > Settings 
> Checkout Pipeline Settigns >
Order Reports.

      

        
        
        
      

    

Hope it helps.