REST service in salesforce (FORBIDDEN)

In the case of a Force.com Site the Apex class must be enabled on the special Guest profile associated with the site.

Setup > Develop > Sites > [your site label] > Public Access Settings > Enabled Apex Classes

This is happening because your user (Profile of the User ) does not have access to the apex class.

Under profiles ,please give access to the myFeedXML class and should work.