How compatible are venv and virtualenv?

I have a lot of Python code tested in environments written with virtualenv and am starting to use venv since several people recommend it and some of my teammates are starting to use it. What are the differences between the two tools in terms of compatibility? I.e., is there a way to know if a specific piece of code that works in an environment created using virtualenv will be equally functional in an equivalent environment created using venv? virtualenv gives problems sometimes, but assuming it does not (I've done extensive testing), I'm wondering whether this ensures proper behavior in a venv environment (or otherwise, is there any rule of thumb on the type of testing required?). See related questions here and here.