If someone contacts you, especially after a long time, you would say 'I'm glad to hear from you' or 'It's good to hear from you after all this time'. This means that you are pleased to have received a call, a letter, a text or an email from this person. 'Hear you' refers to the physical ability to hear this person's voice. For example, on a bad telephone line you would say 'I can't hear you', or when the line improves you might say 'That's better. I can hear you now'.

They can be used interchangeably except that 'hear from you' can mean not only 'listen to your voice' but also 'receive your message/email'.

The correct sentence is "I'm glad to hear from you." when you someone after a long time. But when you're about to leave, you should use "I'm glad hearing from you."