The difference here is that tourism is one kind of travel. All tourism involves some sort of travel, but many kinds of travel do not involve tourism.
Tourism occurs when a person or a group of people travels to a given place to enjoy the things that can be seen and done at that place. This is something that is done for recreation. For example, a person might travel from the US mainland to Hawaii to enjoy warm, beautiful beaches and to see marine life such as dolphins and whales. These people are going to Hawaii for recreational purposes. Of course, this involves travel as the people have to get to Hawaii from the mainland, but what makes it tourism is what they do when they get there.
Travel, by contrast, does not depend on what is done when a person reaches his or her destination. Travel is simply the process of getting to the destination. A person can travel for business purposes. A person can travel to a distant city or country to go to school. Of course, a person can travel for recreation, making that an act of tourism. In this way, tourism is one kind of travel. It is travel for the purpose of enjoying things that can be seen and done at a given destination.