Who is the US President's jury in trial of impeachment?

The full Senate serves as the jury in any impeachment trial.

Most impeachment proceedings have been conducted against federal court judges (below the Supreme Court), so the Senate streamlined the trial process to allow the majority of Senators to conduct the chamber's regular business while an "Impeachment Trial Committee" listens to testimony and acts as a panel of judges. At the end of the trial, the committee compiles a report which the full Senate reads before voting for conviction or acquittal.