An easy way to achieve this is to have a div inside your page containing a partial one with a validation summary. When you invoke a controller action using AJAX, it returns this partial, and you can update the summary. Example:
$('#validationSummary').load('/home/someaction');
where the action will return the view:
public ActionResult SomeAction()
{
return View();
}
and corresponding view:
<%= Html.ValidationSummary() %>
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