I\'m trying to implement events on FOSUserBundle
Two points:
In the service definition - { name: 'kernel.event_subscriber'} is enough, no event entry. This the difference between subscriber and listener. A listener can only listen to one event, defined trough event and method params in the service definition. A subscriber can listen to multiple events, defined trough the method getSubscribedEvents(), so no further params in the service definition needed.
Second point, you are type hinting against the event classes (UserEvent andFilterUserResponseEvent), but you never imported them, so PHP assumes them in the same namespace (as said in the error message).
use FOS\UserBundle\Event\UserEvent;
use FOS\UserBundle\Event\FilterUserResponseEvent;