Doctrine2toXML.php

<?php

// Propel way
$books = BookQuery::create()->find();
$xmlBooks = $books->toXml();

// Doctrine way
// $this is a symfony controller having access to the DIC
$em = $this->get('doctrine.orm.entity_manager');
$books = $em->createQuery('select b from \Acme\DemoBundle\Entity\Book b')->getResult();
$serializer = new \Symfony\Component\Serializer\Serializer();
$serializer->addNormalizer(new \Symfony\Component\Serializer\Normalizer\GetSetMethodNormalizer());
$serializer->setEncoder('xml', new \Symfony\Component\Serializer\Encoder\XmlEncoder());
$xmlBooks = $serializer->serialize($books, 'xml');

But the serializer needs to be defined as well in the DIC, doesn't it?

Yes, I'm not familiar with the Serializer component, but the configuration would look similar.