OS X 10.9 Mavericks is still in development, with Apple only weeks into having released its first preview of the new operating system to its developer and testing communities.
For people who want to install and manage Mavericks in virtual machine environments, Parallels has a new update available that will support the installation of Mavericks in a virtual machine, allowing for testing of code and the new operating system's features without potentially compromising the main operating system installation.
The update is about 325MB in size and can be installed by choosing "Check for Updates" in the Parallels Desktop application menu. Once installed, it should allow you to create a new virtual machine of OS X Mountain Lion and then update it to Mavericks from the Mac App Store within the VM (currently the installation of Mavericks in a freshly created virtual machine is unsupported).
While this update is available for anyone with Parallels installed, you will need a valid developer ID to access the preview build of Mavericks from the Mac App Store.