So I was just wondering what is the difference between Quenya and Elvish.

Generally speaking, Quenya is an Elvish language, like Sindarin for example. The returning Noldor brought a form of Quenya with them when they arrived in Middle-earth after the Rebellion (the Vanyar also spoke a form of Quenya, but they remained in Aman) -- although the Noldor soon enough adopted another kind of Elvish in Middle-earth, usually called Sindarin or Grey-elven by readers and fans (although 'Sindarin' is a Quenya word actually) -- and there are various dialects of Sindarin in any case, so again, generally speaking.

'Elvish' could refer to any of the languages or dialects spoken by the Elves, including the Avarin languages or dialects, about which much less is known compared to Quenya or Sindarin -- which are part of the Eldarin branch of the family tree of Elvish languages.

You're welcome.

And Elvish can also refer to the writing systems of the Elves of course -- thus some who want a tattoo 'in Elvish' might really only mean that they want something in English (or whatever language) written with the Elvish characters -- and people who want the writing seem to usually mean the Tengwar of Feanor, rather than other kinds of Elvish writing.

So Elvish is a broad enough term as used by fans; but to make matters possibly more confusing, Tolkien once wrote with respect to Quenya: 'The archaic language of lore is meant to be a kind of 'Elven-latin', and by transcribing it into a spelling closely resembling that of Latin... the similarity to Latin has been increased ocularly. Actually it might be said to be composed on a Latin basis with two other (main) ingredients that happen to give me 'phonaesthetic' pleasure: Finnish and Greek. It is however less consonantal than any of the three. This language is High-elven or in its own terms Quenya (Elvish)' JRRT, Letter 176

Elsewhere the word Quenya is looked at in more detail, in Quendi And Eldar, in The War of the Jewels for example.

It can be at bit confusing sometimes, and nicely so