Microsoft officials aren't talking about the Xbox One developer story yet; it sounds like they are planning/hoping to hold off until the Build 2013 conference in late June to share more details on that. But here's what I've gleaned from talking to my sources about this.
As we learned yesterday, the Xbox One OS includes a host OS, which is a heavily modified Hyper-V hypervisor; and two partitions. One partition, called the "Exclusive" partition, is a custom virtual machine (VM) that is designed just for games. It is designed to give games on the Xbox One complete control on everything from memory management to storage, I hear. While games can be paused or switched, only one game can run at a time in this VM. (If Microsoft steps up its support for indie games on Xbox One, those games are going to run in this partition.)
The other piece of the new OS the "Shared" partition. It's called shared because multiple applications can share this VM. According to one of my contacts, this VM is based on the Windows 8 "core," which means the kernel, file system, graphics stack, networking stack and security elements. Like the Windows core that is shared with Windows RT and Windows Phone 8, the Shared partition core is based on the WinRT application programming interface (API). One of my sources said internally it's actually called XRT, similar to the way the Windows Phone implementation of WinRT is known internally as WinPRT.
However, the similarities between the Windows environments seem to end there. On top of the Xbox One Shared Partiion core, the Xbox team created a custom UI for Xbox One. Microsoft supposedly isn't going to allow just anyone to write apps that can run here; devs will have to be chosen and invited, the way that they are now on Xbox 360. And they're not going to be inviting Salesforce or Oracle or even the Microsoft SharePoint team to write apps to run here. More likely are things like Netflix and Skype and other media/companion/social kinds of apps.
Core first-party, Microsoft apps like Xbox Music and Video will run in the Shared Partition, too. But these won't just be the same Windows 8/Windows RT versions of these apps; instead, they'll be Xbox-customized ones that will make use of chunks of the Windows 8/Windows RT complements' code, according to my source. (There need to be customized to work with the Xbox controllers and Kinect.)
I'd think as Microsoft continues to add more features and functionality to Windows 8, starting with Windows 8.1 (Blue), some of these security/reliability/performance-focused features will find their way back into the Shared Partition Core. So yeah... don't be looking for the new Start Button or a way to boot straight to desktop on your Xbox One, even when these options come to Windows 8.1.
So at Build 2013, I do expect Microsoft to play up the message that there's increasing a common core codebase on Windows Phone, Windows PCs and tablets and the coming Xbox. There will no doubt be more on the increasingly common developer platform for all of these screens. This year won't be the year when there is just one shared developer platform across these screens (even if Microsoft officials go so far to claim this is the case). Nor will there be a unified Windows Store for Windows 8, Windows RT, Windows Phone 8 and Xbox One this year.
It's taking a while to turn the ship and more closely align the release cadences across all the teams that develop anything that involves a flavor of Windows. Yet slowly and surely, Microsoft's moving in that direction.