By default the XsltListViewWebPart does not support export from the web user interface. I often build solutions by configuring the web parts in the browers, exporting and adjusting the defintions and then deploying them with PowerShell. With a few lines of PowerShell this capability can also be enabled for the Xslt List View Web Part.
Before enabling the Export Mode on the webpart:
After running the PowerShell script and enabling the Export mode: