what model of 3500 XL?
you can connect a 3500 XL series to a 3560 via CAT-5E, CAT-6 or SFP to SC(gbic) fibre.
(the SFP to SC(gbic) option is available if you have a 3500XL with gbic ports such as the 3508XL)
you could also probably use an SFP transceiver if you can find one for the connection type you need to make.