Get IV from Key, Clear and Cipher text in AES-256 CBC Newbie question here but can I get the IV if I know the Key, plain text and chiper text ?

(I thought that if I encrypt 16 first bytes of clear with the key and 16 first bytes of cipher text as IV it would work to get original IV but it seems that it doesn't)

Thanks.
The IV is XOR'ed with the plaintext before encrypting the first block. You can get the input of the block cipher by decrypting the first block of ciphertext using the block cipher directly.

After that it is a simple operation to get the IV, of course, considering you know the plaintext and - hopefully - the mathematical properties of XOR.