Depending upon what version of SharePoint you are using you might be able to this with user roles in InfoPath (if you are using browser forms you will need SharePoint 2010 as the user roles don't work in 2007 browser forms).
However, users would still be able to download the actual XML to their desktop and open in notepad to view so you can't totally prevent someone from digging into that file.
You could try to use views in SharePoint to show users only the data that is assigned to them or that they can contribue to (using the [ME] filter in SharePoint views) however if they dig around they can find the other documents.
If this sort of permissions model does not work for you we have a tool called DBXL which allows you to store the XML in SQL instead of SharePoint and then you can have item level permissions and even prevent users from downloading the XML if they are not authorized to. The tool also includes submit to SQL capabilities which if you have survey forms might help automate the reporting.
You can find out more about DBXL here: http://www.qdabra.com/en/products/DBXL.aspx
If you are intersted in learning more about DBXL feel free to send me a private message via the forums and I can setup a live demo to show you how the tool works.
Ted