How were Luther, Calvin, and Zwingli different?
First, here is an overview of some of the views the three men espoused.
Calvin: He believed that people were born into sin but could be saved by God's grace if they were "elected," which was drastically different from the teachings of the Catholic Church. He also believed that they could not lose that salvation. He believed in the separation of church and state and in sending Christians out as missionaries. He viewed the bread and wine of the Eucharist to be spiritually the body and blood of Christ.
Zwingli: He sought to change how church services were conducted. Rather than a traditional mass, a Communion service took place in churches. He disagreed with the idea of purgatory. He also believed that Christ's actual blood and body were not present in the Eucharist elements and were merely symbols.
Luther: He believed that salvation came from faith and not from a person's good works or their church membership. He disagreed with the Catholic Church selling indulgences and questioned the authority of the Pope. He promoted the idea of Christians performing good works. He agreed with the church on the Eucharist, and believed in transubstantiation (that the elements become the literal body and blood of Christ).
Luther had differing views from the Catholic Church, but he did not seek to completely separate himself from it. Instead, he sought change. For example, he still promoted good works and believed in the transubstantiation. Calvin separated himself from the church with his drastically different beliefs. He believed that people were saved by their faith, with much less of an emphasis on good works. He believed that the bread and wine were spiritually the body and blood of Christ. Zwingli also separated himself from the Catholic Church. He focused on changing how church services were conducted. He believed that the wine and bread were symbols of the blood and body.