yes, see the user manual on select io. Some of the older fpga's only have dedicated pins for differential in, differential in clock, and differential out. newer device tend to allow differential IO, but input clocks are still a special case.
You can do this in pin package. Goto assign pin package or IO plan ahead....
Each pins can be directed to the required IO standard, but not sure about clock standards....
Hi Buddy, I am not sure about values. An easiest way is to download Xilinx Spartan 3 datasheet and check the electrical characteristics. Maybe you should get the answer in minutes....