The original USENIX 2005 Paper by Fabrice himself. Several of the principles described there have survived even to modern versions.
A recent small walkthrough in the source.
A presentation about Qemu Internals.
A presentation about KVM architecture in Linux Kongress 2010
A paper about qemu internals with a view on porting qemu to Plan 9.
A google group post about qemu internals and goldfish (android emulator)
First benchmarks on KVM.
A (master's) thesis about porting Qemu to minix v3
Another thesis about using LLVM as Qemu's dynamic code generator
(instead of current Qemu-specific TCG).
A (2008) USENIX paper on using qemu to automatically reverse engineer device
OLS 2010 paper on porting KVM to ARM.
Slides from a presentation at xensummit 2006 talk about device models
An interesting paper about dynamically translating from x86 back to LLVM
bytecode, using Qemu
Qemu Forks and Patches
SerialICE: A system debugger in software as a modification of Qemu
Qemu in Security Work
integrity checking in qemu. Related is this project.
TTAnalyze, .a tool for analyzing malware, is built on top of Qemu
"An API for virtual I/O: virtio" lwn article.
the current spec etc.
A (relatively old but somewhat relevant) walkthrough on the virtio code.
IBM developerworks article on VirtIO.
Other device technologies
SPICE presentation in KVM Forum 2010, includes info about QXL graphics.
General Virtualization issues
Slides from a university course talking about virtualization in general, Qemu (old) also
A paper on "Subverting Linux on-the-fly using hardware virtualization technology" (information about VT hardware, e.g., VMXON/VMXOFF)
The above list is definitely not exhaustive, if you have links to interesting developer-oriented information about qemu I would be most happy if you would share it with me in the comments :)
Happy Hacking :)