The REST Web Service in Denodo can represent the data as HTML, JSON or XML, this can be set on the Settings tab when you the Edit Web Service.
The data has to be returned by the Web Service in some defined structure, whether it is HTML, JSON or XML. The JSON or XML representations focus on describing the data only, whereas the HTML representation also describes how the data should be formatted in a browser.
HTML is usually the default representation when you define a REST web service and the JSON and XML representations are also published by default (although this can be changed).
If you would like to receive the data in JSON or XML formats add the $format=JSON or $format=XML parameter to the end of the URL for the web service e.g.: for an 'address' web service exposing an 'address' view in the 'admin' VDP database
This is described fully in the Virtual DataPort Administration Guide in the section entitled PUBLISHING REST WEB SERVICES. You can download the guide here:
Hope this helps.