We investigate a partially blind channel estimation and symbol detection in an amplify-and-forward two way relay scheme. We derive generalized likelihood ratio test detectors for reciprocal and non-reciprocal channels, whose computational complexity (CC) grow exponentially with the block length. Therefore, we propose a convergent iterative method for joint channel estimation and symbol detection with CC proportional to the block length. We also address the convergence proof. We further propose the simple blind and partially-blind approaches based on least squares (LS) and weighted LS estimation, respectively, for operation at low SNRs and analyze their mean square errors (MSE). Our simulations reveal the effectiveness of the proposed method compared with its counterparts.