How can I filter http requests by type in Safari 6 Developer Tools?
In the Instruments panel (the icon of the stopwatch), click Network Requests on the left. You should then see the timeline on the left, and a list of requests underneath. Click the Type column heading to sort the requests by type and then all the XHR requests will be grouped together.

Alternatively, in Safari's Develop menu, you can select "Use WebKit Web Inspector" to switch back to the old inspector.