With just a few hours until Apple's keynote, a series of trademark filings for the name 'tvOS' have been discovered by MacRumors, indicating that the Apple TV's new operating system will indeed be referred to as 'tvOS', which would be in line with the naming convention for iOS and watchOS platforms.
Apple appears to have protected the trademark via a shell corporation, as is its usual modus operandi when it wants to hide trademark filings from general public.