Seems there are some different definitions on the documentations what TVs are.
The definition, what comes nearest to my understanding is this one:
http://rtfm.modx.com/display/MODx096/What+are+Template+Variables
found on Evo 096 Documentation.
A Template Variable (TV) is an extra or custom Resource Field that is created by the site developer.
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TVs are template specific, meaning they can only be used in templates that they are assigned to.
For me all fields, which are in the site_content - table are resource-fields.
They are sometimes called also standard-template-variables.
In any case, in this context with getResources and tvFilters they aren’t TVs, or at least, you have to differentiate between standard-template-variables and custom-template-variables.
With tvFilters you can only filter by real TVs (custom-template-variables), which are stored in the site_tmplvar_contentvalues - table.
For resource-fields or standard-template-variables, if you want so, you need the &where - property for filtering.