Difference between learning_curve and validation_curve
Both curves show the training and validation scores of an estimator on the y-axis.

A learning curve plots the score over varying numbers of training samples, while a validation curve plots the score over a varying hyper parameter.

The learning curve is a tool for finding out if an estimator would benefit from more data, or if the model is too simple (biased).



Above example shows the training curve for a classifier where training and validation scores converge to a low value. This classifier would hardly benefit from adding more training data; a more expressive model may be more appropriate.

The validation curve is a tool for finding good hyper parameter settings. Some hyper parameters (number of neurons in a neural network, maximum tree depth in a decision tree, amount of regularization, etc.) control the complexity of a model. We want the model to be complex enough to capture relevant information in the training data but not too complex to avoid overfitting. 



Above example shows the validation curve over a support vector machine's gamma parameter. A too low value of gamma restricts the model too much; both, training and validation scores are very low. A high value of gamma causes overfitting: very good training score but low validation score. The optimal value is somewhere in the middle, where the curves do not diverge too much.

Image source: scikit-learn documentation