I need to receive 42 pairs of LVDS signals to a FPGA. All i want is that the FPGA converts the differential pairs in normal std_logic signals. And i need to choose a xilinx FPGA with enough pins that support LVDS signaling. I'm a bit confused and don't know which kind of FPGA to choose depending on the standards I/O LVDS or LVDS_25 I don't really know how the standards work or how do they convert the signals Any help would be much appreciated Thank you
LVDS and LVDS_25 are most likely electrically the same. I don't know which family you're looking at, but in the Spartan-6 there are also two LVDS standards, LVDS_25 and LVDS_33. They are electrically the same (see below from DS162), so they will work the same as far as interfacing with other LVDS parts. The distinction is that you use LVDS_33 when your bank voltage is 3.3V and LVDS_25 for 2.5V.
You can confirm this by looking at the input and output levels in the datasheet of the part you're evaluating.