Is IGBT a thyristor?

No. IGBT and thyristor are different semiconductor devices.

The thyristor is a four layer ( P-N-P-N ) semiconductor device that contains three PN junctions in series, and is represented by the symbol as shown below:

The Insulated Gate Bipolar Transistor also called an IGBT for short, is something of a cross between a conventional Bipolar Junction Transistor, (BJT) and an MOS Field Effect Transistor, (MOSFET) making it ideal as a semiconductor switching device: