Here is an example of using the user provider to load a user check the password matches then setting the token in the security service. So if you put this code into a route you can get access to the Request for your username and password.
$userProvider = $app['security.user_provider.default'];
$user = null;
try {
$user = $userProvider->loadUserByUsername($username);
} catch (UsernameNotFoundException $e)
{
;
}
$encoder = $app['security.encoder_factory']->getEncoder($user);
// compute the encoded password
$encodedPassword = $encoder->encodePassword($password, $user->getSalt());
// compare passwords
if ($user->password == $encodedPassword)
{
// set security token into security
$token = new UsernamePasswordToken($user, $password, 'yourProviderKeyHere', array('ROLE_USER'));
$app['security']->setToken($token);
// redirect or give response here
} else {
// error feedback
}