By default, when you create a Canvas App and try to access the form fields of an entity record, these fields will not be present in the designer:

 

To do this you need to add first the Canvas APP in the form. Copy your Canvas APP Id in the Details section:

 

And paste it in the form editor:


 
The Entity name field will auto populate the entity logical name of the form, in this case Contact.

Then click the Customize button. This will be open a new window to edit the Canvas APP but now with a new component: ModelDrivenFormIntegration.
 
Then select the control you need to use to show the field value, for example fullname. In the Text function put “[@ModelDrivenFormIntegration].Item.'Full Name'”. But this will throw an error because we need to specify to ModelDrivenFormIntegration the DataSource. To do this add the Entity in the Data section:

 

And map that in the DataSource property of the ModelDrivenFormIntegration.
 
And now you will see the values correctly:

 

Save and publish the Canvas APP.
Save and publish the form.
 
Enjoy it!