Screenshots of two Windows builds that are known internally as Windows Blue, have been leaked, which reveal that Microsoft has bumped Windows kernel to 6.3, where as Windows 8 uses kernel 6.2.
Windows Vista used NT kernel 6.0, while Windows 7 jumped to 6.1, and Windows 8 to 6.2. A switch to 6.3 with Windows Blue suggests this is a major revision to Windows.
Source: MicrosoftPortal.net, via The Verge