HMD...means Head Mounted Display, not Virtual Reality headset. They are personal video viewer. How is that even related to VR? It's like comparing a digital photo frame and a tablet.
I really wish people would stop trying to make this false distinction. Most VR devices are HMDs. Including oculus.
http://en.wikipedia.org/wiki/Oculus_Rift
The Oculus Rift is an upcoming virtual reality head-mounted display. It is being developed by Oculus VR, which has raised US $91 million, of which $2.4 million was raised with crowdfunding via Kickstarter.[5] The company was founded by Palmer Luckey and Scaleform co-founders Brendan Iribe and Michael Antonov,[6] as well as Nate Mitchell. id Software co-founder John Carmack was later hired as its Chief Technology Officer.[7]