No such thing is available. While it is true that Emacs can by now display PDF files in a separate window, that is not really feasible for anything but quite small documents, like 1 or 2 pages. You are missing a lot of features from a proper PDF viewer, though, and it has considerable memory impact depending on document size. AUCTeX has no interface to that functionality, though you can just load the PDF file into a separate split window with
C-x 4 f.
What AUCTeX does offer is preview-latex: this will not create a split window. Instead, selected graphical elements (typically images and math) will be replaced right in the buffer with graphics, and you can run the cursor (left/right) into them in order to get the source text back. While its use is more limited than that of a full-bodied preview (for example, it is useless for checking the vertical page layout), access is much more direct and intuitive than even a split window. Try
C-c C-p C-p and/or the Preview menu. It is conceivable that you need to follow installation instructions for making it available, depending on the version of AUCTeX installed on your system.