Create a form as a service in Symfony2
$regsiter = $this->get('register_manager');
return $this->render(
    'AcmeUserBundle:Account:register.html.twig',
     array(
         'form' => $regsiter->getRegistrationForm(),
         )
 );

<form>
    {{ form_widget(form) }}
</form>

parameters:
    register_manager.class: AcmeUserBundleManagerRegisterManager

services:
    register_manager:
        class:     %register_manager.class%
        arguments: [@form.factory]

namespace AcmeUserBundleManager;

use AcmeUserBundleFormTypeRegistrationType;
use SymfonyComponentFormFormFactoryInterface;

class RegisterManager
{

    protected $formFactory;

    public function __construct(FormFactoryInterface $formFactory)
    {
        $this->formFactory = $formFactory;
    }


    public function getRegistrationForm()
    {
        return $this->formFactory->createBuilder(new RegistrationType());
    }
}

namespace AcmeUserBundleFormType;

use SymfonyComponentFormAbstractType;
use SymfonyComponentFormFormBuilder;

class RegistrationType extends AbstractType
{
    public function buildForm(FormBuilder $builder, array $options)
    {
        $builder->add('username','text');
        $builder->add('email','email');
        $builder->add('password','password');
    }

    public function getDefaultOptions(array $options)
    {
        return array(
            'data_class' => 'AcmeUserBundleEntityUser',
        );
    }

    public function getName()
    {
        return 'registration';
    }
}