1) Is it necessary to expose a webservice at UDDI if both the service and client know each other,I mean both the client and service are in same organisation but run on different servers ?
Necessary? No. Advantageous? Possibly, depending on the circumstances.
2) If a Web service is implemented using jaxws and want to expose it , Does it should to register it at UDDI or somewhere else ?
Unless the circumstances are more complicated than what you're describing, then, generally speaking, no.
The client who wants to use this service have to generate the client stubs using the wsdl provided by implemented service,..am I right ?
Yes, the WSDL is the way to describe a WS.
3) If I am writing a service and client myself,that means the client knows the location of server,in this case do I still need registering with UDDI ?
No, there are very few scenarios where using UDDI is advantageous. Although it is still used here and there -mainly internally in large organizations- it largely failed to establish itself in the market.