VT-d = Intel Virtualization Technology for Directed Input/Output
Intel VT-d allows paravirtualized operating systems to access hardware devices on the host operating system directly. It eliminates hardware device emulation and improves I/O performance for the virtual machine tremendously. Intel's VT-d technology is analogous to AMD's IOMMU.
Article: Intel® Virtualization Technology for Directed I/O (VT-d): Enhancing Intel platforms for efficient virtualization of I/O devices