I'll try to reading the source code of qemu, it's about 20MB in total, of the latest version 0.10.5.
Firstly, the documents reading needed, here is the docs on it's official website: http://www.qemu.org/user-doc.html
Then, be familiar with the architecture of the source code.
1) I'm testing QEMU+KVM at work time, and I'm a little familiar with it.
2) I wana to get some usage of system call
3) We can get almost knowledge of every aspect from the virtual machine simulator, the CPU, the hardware, the kernel, the syscall, etc.