Create dynamic wordrpess blank page

Try something like this: in the load hook, you can ouput your xml code.
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function my_menu_pages() {
    $hook = add_submenu_page(
            null, 'Page Title', 'Page Title', 'administrator', 'sub-menu-slug', function() {
    }
    );
    add_action('load-' . $hook, function() {
        // add your xml code here, 
        // you will get a blank page to start with
        exit;
    });
}

add_action('admin_menu', 'my_menu_pages');