Q: 

Are the EFM32 Starter Kits powered by the USB or the battery.

A: 

The power source for the Starter Kits is controlled by the switch next to the battery slot. The EFM32 Gecko Starter Kit works slightly different from the EFM32 Tiny Gecko Starter Kit.

Tiny Gecko STK:

When the switch is pulled towards battery connector, the kit is powered by the battery. 

When the switch is pulled away from the battery connector, the kit is powered by the USB power.

Gecko STK:

When the switch is pulled towards battery connector, the kit is powered by either the battery or the USB power, depending on which voltage is higher (USB gives 3.3 V). 

When the switch is pulled away from the battery connector, the kit is powered by the USB power.

It is also possible to power the EFM32 devices on all ktis by directly applying power to one of the VMCU breakout points.