Google search gives about 19,500,000 results for "haven't known" and about 12,500,000 results for "haven't knew". So I am a bit confused about this.
Could anyone please explain how should I say and why?
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Have is an auxiliary verb. When we use have, we should use the past participle form of the verb. In your example, the past participle of know is known, not knew.
Seeing the number of Google results may be one way of knowing the truth. However, since we are talking about grammar here (which has rules), there are lots of good references in the Internet that can help you.
First of all you must have searched for haven't knew without quotes. This will give you results that have those two words but not necessarily together.
Try searching for "haven't knew" but include the quotation marks. There are still many results but not nearly so many. Most are nonsense or written by non-natives or poor attempts at humour.
Instead of searching Google, which has all sorts of errors, you should search published books and papers. You can do this with, for example, Google ngram.
Google ngram: haven't known,haven't knew
If you do this you can see that there is not one single example of 'haven't knew'.
As Lester Nubla says, just as with any other verb, you must use the past participle after the auxiliary verb 'to have'. You will find that in any good dictionary.