What are the differences between the Virtex-E voltage-mode driver and the Virtex-II current-mode driver?
From a signaling standpoint, there is no detectable difference -- both implementations adhere to the LVDS standard. Please see the "DC and Switching Characteristics" section of the appropriate device data sheet for specifics. The Xilinx FPGA data sheets are available at:
From an architecture/topology standpoint, the difference is that you do not have to use external termination at the Virtex-II driver interface. However, external driver termination is required for Virtex-E. Please see Xilinx Application Note "Using the Virtex Select I/O Resource" (Xilinx XAPP133) for Virtex-E termination guidelines.
From an electrical standpoint, there is an improvement in the simultaneous switching output (SSO) characteristics of the current-mode driver in Virtex-II because the positive and negative sides of the differential pair are both driven from a single current source. Since this source passes constant current, there is only a miniscule transient current when the differential pair switches. In Virtex-E, slight skews and differences of voltage swing at different drivers mean that the currents from the two sides of a differential pair will not cancel each other perfectly. This results in a small amount of transient current.
Please refer to (Xilinx Answer 13572) for information on Virtex-E/II LVDS SSO guidelines.