I am not sure that I completely understand your scenario, but please consider the following information and if it does not help please send us a fully working project (you must open a support ticket to be able to attach files, or you can paste the full source code in this thread) that demonstrates the problem and we'll do our best to provide a working solution.
The XmlHttpPanel uses client callbacks (or WebService) to update its content. This means that the page will not go through its full life-cycle when its content is loaded, as in the case with AJAX postbacks. Additionally there is no ViewState, so if you perform a postback the content inside the XmlHttpPanel might be lost (or inconsistent). I have created a sample project (attached to the thread) to demonstrate this. It has a GridView which is updated by the XmlPanel on a button client-click. Then after a postback to the server the content of the GridView is lost, because there is no ViewState.
For additional info read the XmlHttpPanel's online documentation
the Telerik team