$message = message_load($mid, $view_mode);
$output = drupal_render(entity_view($message));

$build = $message->buildContent('my_view_mode');
$output = drupal_render($build);

$message = message_load($mid);
$build = $message->view();
$build += array(
  '#theme' => 'message',
  '#entity' => $message,
  '#view_mode' => 'full',
);
$output = drupal_render($build);