Starting from Xena Valkyrie software release 77 the chassis supports REST Server in addition to the CLI API.
The REST server is enabled and running on port 57911 by default. No need to install or start anything. Just go to URL http://chassis IP:57911 and start your REST session.
To get to the starting point of Xena REST server just go to:
http://Chassis IP Address:57911
If you set this URL in a browser, you will get to the swagger UI:
Here you can see all namespaces and try out all REST commands to get to know the API better.
If you browse to http://Chassis IP Address:57911/swagger.json you can get the swagger file behind the server.
The latest version of xenavalkyrie python package supports REST API. You can download it from pypi.