According to at least two different sources, the Xbox One games Microsoft showcased during E3 2013 were not running on an actual console or even on a development kit. Instead, it seems that those games were running on a Windows 7 PC equipped with a NVidia graphics card.
The news was confirmed by US Gamer’s Julian Rignall who twitted that he has "just played an Xbox One game using an Xbox One controller that crashed... to a Windows 7, Hewlet Packard-branded desktop. Magic!"
Additionally, a NeoGAF forum member managed to snap a photo of a PC equipped with NVidia GTX 700 graphics card hidden inside one of the Xbox One demo kiosks. For your convenience, the photo is available in the gallery.
It is worth noting that Xbox One is powered by AMD CPU and GPU and that its operating system is based on Windows 8 core.