The installers don't put anything on PATH by default. Activestate installers and my custom installers do put PythonNN and PythonNN\Scripts on the path.

It's true that PythonNN\Scripts is the place that third-party command-line tools are installed. I don't believe Python itself installs anything there. However, py.exe is installed in C:\Windows and python.exe is installed in PythonNN.

So, to be consistent with the installation of py.exe, pyvenv should be installed in C:\Windows. That would not work well because pyvenv should be specific to a particular Python version.

That's why I suggested the PythonNN directory, because it's part of the core Python installation, where python.exe is installed, and isn't a third-party script. That said, PythonNN\Scripts also seems like it might be a good location. It would necessitate adding that to the PATH, which is a reasonable thing to do. The only case where that wouldn't work is if someone wanted to have pyvenv in the path but not the third-party scripts installed to PythonNN\Scripts.