•what is the lacking's of developing sharepoint environment in VM ?
nothing much appart from sharing your pc resources like ram :). Its a bit of a pain to setup but once setup you wont look back!
•what is the benefit of Developing in VM?
You can break sharepoint and server and it wouldnt matter as it doesnt effect anyone appart form you. You can create a backup of your vhd and/or create copies to run multiple versions at the same time.
•What is the benefit of developing in physical machine?
same benifit as vm appart from making mistakes what do happen and would be pain to redo/setup everthing if somthing bad does go wrong!
•what are the lacking's or worst scenario's of developing in Physical
Machine ..?
developing in physical machine would mean that you dont have room for mistakes. If somthing very bad goes wrong you would either need to solve the issue (takes time) or reinstall (takes time) with vm you have copies or backup.
So with vm its highly recommended and I use it for development every day! I have two copies that i can run if i want to but its mainly for emergancy perpouses like if one goes dead then i can run the other. I also have a backup of my vhd just incase. I always save the machine state when closing the vm for fast boot with everything that i was working on already up! booting takes time otherwise! having vm and your local machine has better advantages over it being on physical drive.
EDIT
Making vm faster
To make your vm faster have a faster cpu and alot of ram, I would expect 6gm as good but 8gb is preffered. You can have 4gb but will be slow when running loads of apps.
when setting up vm make sure that you give nomore than half your ram otherwise it will slow your main os so slow that it might even crash due to bottle neck.
I give mine just shy of half. Try not to load many apps as i do as it will slow your vm and pc! i currently have 5 vs2008 running with 3 ie open, 1 stsadm, 1 iis and 3 folders open and thats just on my vm! try not to clutter your desktop on both os with shortcuts and files as it will add to the explorer.exe as it has to make loads of icons that can slow you down slightly everytime you goto desktop!
make sure there is plenty of room in your hdd (hard disk drive) for code, vs2008 - 2010, sharepoint ect... also when the hdd on your main os is nearing to full it can also hinder on speeds!
NEVER shut down your vm!!! by that i mean never power off the state, always save the machine state. This way when you open the saved state it will load within seconds and it will load exactly the same state you left it in, so if you had vs2008 with some ie open it will open it with them open! if you shutdown the state then it will take ages to load backup again!
for more tips check this out :)
http://www.hanselman.com/blog/VMPerformanceChecklistBeforeYouComplainThatYourVirtualMachineIsSlow.aspx
btw i have core i3 and 6gb ram with nvid geforce 310m on win 7 64bit os. its a laptop not desktop as i can also work from home :)
hope it helps :)