Hardware-accelerated H.264 encoding on NVIDIA Kepler and Maxwell cards is available for a number of VDI and similar solutions.
The current XenDesktop version 7.8 supports NVENC for Linux VMs with its HDX 3D Pro VDA but not Windows OS VMs. Versions 7.7 and lower do not support hardware accelerated H.264 encode. See Citrix announcement for details of supported Linux OSs: https://www.citrix.com/blogs/2016/04/04/virtualize-linux-3d-applications-with-hdx-3d-pro-for-linux/. Support may change with future versions.
Update: Citrix XenDesktop 7.11 introduced support for Windows as well as Linux OSs with support for multi-monitors on both Linux and Windows OSs. You can read about the HDX/XenDesktop 7.11 enhancements here: https://docs.citrix.com/en-us/xenapp-and-xendesktop/7-11/hdx/gpu-acceleration-desktop.html
The current version of Horizon 7.0.1 supports NVENC for Windows OS only and not Linux VMs. Blast Extreme protocols support CPU based H.264 encode only for Linux VMs. Support may change with future versions.
Nice DVC supports NVENC Hardware-accelerated H.264 encoding on NVIDIA GRID cards for both Linux and Windows OSs.
HP RGS supports NVENC Hardware-accelerated H.264 encoding on NVIDIA GRID cards for both Linux and Windows OSs.
NVIDIA GRID GPU and vGPU enabled VMs
Linux (Ubuntu, SUSE, RHEL, CentOS)
NVIDIA GRID GPUs including K1, K2, M6, M6, M10