What is the difference between SV17 and SV19 bicycle tubes? I know that "SV" is short for "Sclaverand valve" and this page shows a really nice schema for the difference between AV / SV / DV. Especially the diameter of the valve seems to be set by AV / SV /DV.

The diameter of the tube as a whole is set by the notation "28 x 1.40" where "28" means 28 inch (I'm not sure about the 1.40... maybe the inner diameter of the tube".

But then I see "SV17" and "SV18" ... how are they different?

The question is specific about Schwalbe inner tubes as only they seem to use that notation.
SV17 and SV19 are quick identifying codes for Schwalbe tubes. I have reproduced an extract below from Schwalbe's Website. I'm assuming they won't mind for the advertising benefit. 

From the table we see that each model of tube they make is given an alphanumeric code starting with AV, DV and SV which you realise are for Auto Valve (Schrader), Dunlop Valve (Woods) and Sclaverand Valve (Presta) respectively, colour coded for easy identifying. Each size of tube is then given a number like 17, 19, so we can look up in the table what tyres it is suitable for. Or more likely, look up what tube best suits our bike and find it on the shelf quickly and easily. Or keep said shelf tidy and organised and re-order from suppliers.

SV17 and SV19 do overlap in some of their size ranges, e.g. they would both fit a 40-622 tyre, but SV19 also covers some 27.5, and 29er sizes and is quite a bit heavier.

The major 'gotcha' to be aware of is that someone (eg an English speaker) wanting a Schrader valve tube should be careful to order an AutoValve (AV) tube not a SclaverandValve (SV) tube, by remembering that Schwalbe is a German brand.