Is it possible to restrict a route for AJAX only?

I know this question is a bit older but meanwhile a new way to achieve this was introduced in Symfony 2.4.

Matching Expressions

For an ajax restriction it would look like this:

contact:
    path:     /contact
    defaults: { _controller: AcmeDemoBundle:Main:contact }
    condition: "request.isXmlHttpRequest()"

Also possible in Annotation:

/**
 * ContactAction
 *
 * @Route("/contact", name="contact", condition="request.isXmlHttpRequest()")
 */

My advice would be to define your own router service instead of default, which would extend from Symfony\Bundle\FrameworkBundle\Routing\Router, and redefine method resolveParameters() with implementing your own logic for handling additional requirements.

And then, you could do something like this in your routing:

your_route:
    pattern:  /somepattern
    defaults: { somedefaults }
    requirements:
        _request_type:  some_requirement