JIT compilation is a model of execution which translates at run time critical parts of the program to a low level representation. Typically a JIT compiler produces machine code from an intermediate bytecode representation. This paper considers a hardware JIT compiler targeting FPGAs, which are digital circuits configurable as needed to implement application specific circuits. Recent FPGAs in the Xilinx Virtex family are particularly attractive for hardware JIT because they are reconfigurable at run time, they contain both CPUs and reconfigurable logic, and their architecture strikes a balance of features. In this paper we discuss the design of a hardware architecture and compiler able to dynamically enhance the instruction set with hardware specialized instructions. A prototype system based on the Xilinx Virtex family supporting hardware JIT compilation is described and evaluated.