I have to test a custom Android build on a tablet and I need to know the method to debug the Android kernel from booting in the tablet?
The tablet has only a screen, 2 USB ports and an HDMI port.
Generally, what you need to achieve this is to connect to the serial console of the system. Like on a desktop system, this gives you all the kernel and init messages as the device boots, and access to a shell once the device has booted.
How to connect to the serial console varies between devices, and you haven't told us which device you have. Usually, you need to take the main board out of its case (this is irreversible in some devices, as you have to damage the case to get it open), and find the serial port. In most cases, there won't be any connector there, just exposed points on the motherboard to solder a connector or ribbon cable to. You need to find the datasheet for the device online to find the right point to connect to.
On some devices, the serial console is instead provided by connecting a special adaptor to the headphone jack. With a proper build of the kernel, the device can be made to send serial output on that connector, and you can connect to it with an adaptor you make yourself (if you can get the specifications from the manufacturer).