What is basic structure of xml for importing a page in wordpress?

WordPress has a full load of Importer Plugins that you can install from the ~/wp-admin/tools/import screen. They will serve as nice reference.

Then there’s Ralf Alberts @Ralf912 Importer skeleton on GitHub/Gist which you can take as reference. Please note that this skeleton uses namespaces and therefore requires PHP 5.3+, or a rewrite.

The main/important parts are the following:

1. Check if the request comes from admin.php?import=...

   if ( ! defined( 'WP_LOAD_IMPORTERS' ) )
       return;
   

2. Load Importer API (Importer API = Backendpage admin.php?import=...)

   require_once ABSPATH . 'wp-admin/includes/import.php';
   

3. Load the importer base class

   if ( ! class_exists( 'WP_Importer' ) ) {
       $class_wp_importer = ABSPATH . 'wp-admin/includes/class-wp-importer.php';
       if ( file_exists( $class_wp_importer ) )
           require $class_wp_importer;
   }
   

4. Register your Custom Importer to WordPress by using register_importer() – Codex explanation

   register_importer(
       'custom',
       'CustomPress',
       __('Import <strong>posts</strong> or not.', 'custom-importer'),
       array(
           new Custom_Import(),
           'dispatch'
           )
   );